diff --git a/Build/build-ai-cidr.ts b/Build/build-ai-cidr.ts
index d9031b77..615b91e7 100644
--- a/Build/build-ai-cidr.ts
+++ b/Build/build-ai-cidr.ts
@@ -2,7 +2,7 @@ import { fastIpVersion } from 'foxts/fast-ip-version';
 import { SHARED_DESCRIPTION } from './constants/description';
 import { $$fetch } from './lib/fetch-retry';
 import { RulesetOutput } from './lib/rules/ruleset';
-import { task } from './trace';
+import { SpanCategory, task } from './trace';
 
 const OPENAI_VOICE_IP_URL = 'https://openai.com/chatgpt-voice.json';
 
@@ -67,7 +67,7 @@ export const buildAICIDR = task(require.main === module, __filename)(async (span
   const { cidr4, cidr6, lastUpdated } = await span.traceChildAsync('get OpenAI Voice IP ranges', async () => {
     const response = await $$fetch(OPENAI_VOICE_IP_URL);
     return parseOpenAIVoiceJSON(await response.json());
-  });
+  }, SpanCategory.Network);
 
   return new RulesetOutput(span, 'ai', 'ip')
     .withTitle('Sukka\'s Ruleset - ChatGPT Voice IP CIDR')
diff --git a/Build/build-apple-cdn.ts b/Build/build-apple-cdn.ts
index cc205ffd..ef544af9 100644
--- a/Build/build-apple-cdn.ts
+++ b/Build/build-apple-cdn.ts
@@ -1,5 +1,5 @@
 import { parseFelixDnsmasqFromResp } from './lib/parse-dnsmasq';
-import { task } from './trace';
+import { SpanCategory, task } from './trace';
 import { SHARED_DESCRIPTION } from './constants/description';
 import { DomainsetOutput } from './lib/rules/domainset';
 import { $$fetch } from './lib/fetch-retry';
@@ -7,7 +7,7 @@ import { $$fetch } from './lib/fetch-retry';
 const getAppleCdnDomainsPromise = $$fetch('https://raw.githubusercontent.com/felixonmars/dnsmasq-china-list/master/apple.china.conf').then(parseFelixDnsmasqFromResp);
 
 export const buildAppleCdn = task(require.main === module, __filename)(async (span) => {
-  const res: string[] = await span.traceChildPromise('get apple cdn domains', getAppleCdnDomainsPromise);
+  const res: string[] = await span.traceChildPromise('get apple cdn domains', getAppleCdnDomainsPromise, SpanCategory.Network);
 
   return new DomainsetOutput(span, 'apple_cdn')
     .withTitle('Sukka\'s Ruleset - Apple CDN')
diff --git a/Build/build-cdn-download-conf.worker.ts b/Build/build-cdn-download-conf.worker.ts
index 55823a2d..5f668ea1 100644
--- a/Build/build-cdn-download-conf.worker.ts
+++ b/Build/build-cdn-download-conf.worker.ts
@@ -1,6 +1,6 @@
 import path from 'node:path';
 import { readFileIntoProcessedArray, fetchRemoteTextByLine } from './lib/fetch-text-by-line';
-import { task } from './trace';
+import { SpanCategory, task } from './trace';
 import { SHARED_DESCRIPTION } from './constants/description';
 import { appendArrayInPlace } from 'foxts/append-array-in-place';
 import { SOURCE_DIR } from './constants/dir';
@@ -22,8 +22,7 @@ export const buildCdnDownloadConf = task(require.main === module, __filename)(as
     downloadDomainSet,
     steamDomainSet
   ] = await Promise.all([
-    span.traceChildAsync(
-      'download public suffix list for s3',
+    span.traceChild('download public suffix list for s3', SpanCategory.Network).traceAsyncFn(
       async () => {
         const trie = new HostnameTrie();
 
@@ -63,8 +62,7 @@ export const buildCdnDownloadConf = task(require.main === module, __filename)(as
         return S3OSSDomains;
       }
     ),
-    span.traceChildAsync(
-      'load public ipfs gateway list',
+    span.traceChild('load public ipfs gateway list', SpanCategory.Network).traceAsyncFn(
       async () => {
         const data = await (await $$fetch('https://cdn.jsdelivr.net/gh/ipfs/public-gateway-checker@main/gateways.json')).json();
         if (!Array.isArray(data)) {
diff --git a/Build/build-chn-cidr.ts b/Build/build-chn-cidr.ts
index 808927dc..695b7e45 100644
--- a/Build/build-chn-cidr.ts
+++ b/Build/build-chn-cidr.ts
@@ -1,5 +1,5 @@
 import { fetchRemoteTextByLine } from './lib/fetch-text-by-line';
-import { task } from './trace';
+import { SpanCategory, task } from './trace';
 
 import { IPListOutput } from './lib/rules/ip';
 import { createFileDescription } from './constants/description';
@@ -10,7 +10,7 @@ const getChnCidrPromise = Promise.all([
 ]);
 
 export const buildChnCidr = task(require.main === module, __filename)(async (span) => {
-  const [filteredCidr4, cidr6] = await span.traceChildPromise('download chnroutes2', getChnCidrPromise);
+  const [filteredCidr4, cidr6] = await span.traceChildPromise('download chnroutes2', getChnCidrPromise, SpanCategory.Network);
 
   // Can not use SHARED_DESCRIPTION here as different license
   const description = createFileDescription('CC BY-SA 2.0');
diff --git a/Build/build-common.ts b/Build/build-common.ts
index ca3af8e6..844ec71b 100644
--- a/Build/build-common.ts
+++ b/Build/build-common.ts
@@ -4,7 +4,7 @@ import * as path from 'node:path';
 import { readFileByLine } from './lib/fetch-text-by-line';
 import { processLine } from './lib/process-line';
 import type { Span } from './trace';
-import { task } from './trace';
+import { SpanCategory, task } from './trace';
 import { SHARED_DESCRIPTION } from './constants/description';
 import { fdir as Fdir } from 'fdir';
 import { appendArrayInPlace } from 'foxts/append-array-in-place';
@@ -95,7 +95,7 @@ function processFile(span: Span, sourcePath: string) {
     }
 
     return [title, descriptions, lines, sgmodulePathname] as const;
-  });
+  }, SpanCategory.FsRead);
 }
 
 async function transform(parentSpan: Span, sourcePath: string, relativePath: string) {
diff --git a/Build/build-deprecate-files.ts b/Build/build-deprecate-files.ts
index 711299bf..04f73ec2 100644
--- a/Build/build-deprecate-files.ts
+++ b/Build/build-deprecate-files.ts
@@ -1,6 +1,6 @@
 import { OUTPUT_CLASH_DIR, OUTPUT_SURGE_DIR, PUBLIC_DIR } from './constants/dir';
 import { compareAndWriteFile } from './lib/create-file';
-import { task } from './trace';
+import { SpanCategory, task } from './trace';
 import path from 'node:path';
 import fsp from 'node:fs/promises';
 import { globSync } from 'tinyglobby';
@@ -29,7 +29,7 @@ const REMOVED_FOLDERS = [
   'Clash/Internal'
 ];
 
-export const buildDeprecateFiles = task(require.main === module, __filename)((span) => span.traceChildAsync('create deprecated files', async (childSpan) => {
+export const buildDeprecateFiles = task(require.main === module, __filename)((span) => span.traceChild('create deprecated files', SpanCategory.FsWrite).traceAsyncFn(async (childSpan) => {
   const promises: Array<Promise<unknown>> = globSync(REMOVED_FILES, { cwd: PUBLIC_DIR, absolute: true })
     .map(f => fsp.rm(f, { force: true, recursive: true }));
 
diff --git a/Build/build-domestic-direct-lan-ruleset-dns-mapping-module.ts b/Build/build-domestic-direct-lan-ruleset-dns-mapping-module.ts
index f20354cf..864d9807 100644
--- a/Build/build-domestic-direct-lan-ruleset-dns-mapping-module.ts
+++ b/Build/build-domestic-direct-lan-ruleset-dns-mapping-module.ts
@@ -5,7 +5,7 @@ import { DIRECTS, HOSTS, LAN } from '../Source/non_ip/direct';
 import type { DNSMapping } from '../Source/non_ip/direct';
 import { fetchRemoteTextByLine, readFileIntoProcessedArray } from './lib/fetch-text-by-line';
 import { compareAndWriteFile } from './lib/create-file';
-import { task } from './trace';
+import { SpanCategory, task } from './trace';
 import type { Span } from './trace';
 import { SHARED_DESCRIPTION } from './constants/description';
 import { once } from 'foxts/once';
@@ -372,7 +372,7 @@ export const buildDomesticRuleset = task(require.main === module, __filename)(as
 async function buildLANCacheRuleset(span: Span) {
   const childSpan = span.traceChild('build LAN cache ruleset');
 
-  const cacheDomainsData = await childSpan.traceChildAsync('fetch cache_domains.json', async () => (await $$fetch('https://cdn.jsdelivr.net/gh/uklans/cache-domains@master/cache_domains.json')).json());
+  const cacheDomainsData = await childSpan.traceChildAsync('fetch cache_domains.json', async () => (await $$fetch('https://cdn.jsdelivr.net/gh/uklans/cache-domains@master/cache_domains.json')).json(), SpanCategory.Network);
   if (!cacheDomainsData || typeof cacheDomainsData !== 'object' || !('cache_domains' in cacheDomainsData) || !Array.isArray(cacheDomainsData.cache_domains)) {
     throw new TypeError('Invalid cache domains data');
   }
@@ -388,7 +388,8 @@ async function buildLANCacheRuleset(span: Span) {
       allDomainFiles.map(
         async (file) => childSpan.traceChildAsync(
           'download ' + file,
-          async () => Array.fromAsync(await fetchRemoteTextByLine('https://cdn.jsdelivr.net/gh/uklans/cache-domains@master/' + file, true))
+          async () => Array.fromAsync(await fetchRemoteTextByLine('https://cdn.jsdelivr.net/gh/uklans/cache-domains@master/' + file, true)),
+          SpanCategory.Network
         )
       )
     )
@@ -444,6 +445,8 @@ async function buildLANCacheRuleset(span: Span) {
     mihomoOutput.addDomain(domain);
   }
 
+  childSpan.stop();
+
   return Promise.all([
     surgeOutput.write(),
     mihomoOutput.write()
diff --git a/Build/build-microsoft-cdn.worker.ts b/Build/build-microsoft-cdn.worker.ts
index d0c5031a..21a17cbc 100644
--- a/Build/build-microsoft-cdn.worker.ts
+++ b/Build/build-microsoft-cdn.worker.ts
@@ -1,4 +1,4 @@
-import { task } from './trace';
+import { SpanCategory, task } from './trace';
 import { SHARED_DESCRIPTION } from './constants/description';
 import { RulesetOutput } from './lib/rules/ruleset';
 import { RULES, PROBE_DOMAINS, DOMAINS, DOMAIN_SUFFIXES, BLACKLIST } from './constants/microsoft-cdn';
@@ -8,7 +8,7 @@ import { appendArrayInPlace } from 'foxts/append-array-in-place';
 import { extractDomainsFromFelixDnsmasq } from './lib/parse-dnsmasq';
 
 export const buildMicrosoftCdn = task(require.main === module, __filename)(async (span) => {
-  const [domains, domainSuffixes] = await span.traceChildAsync('get microsoft cdn domains', async () => {
+  const [domains, domainSuffixes] = await span.traceChild('get microsoft cdn domains', SpanCategory.Network).traceAsyncFn(async () => {
     const trie = new HostnameSmolTrie();
 
     for await (const line of await fetchRemoteTextByLine('https://raw.githubusercontent.com/felixonmars/dnsmasq-china-list/master/accelerated-domains.china.conf')) {
diff --git a/Build/build-public.ts b/Build/build-public.ts
index 7f410752..84d6ecd0 100644
--- a/Build/build-public.ts
+++ b/Build/build-public.ts
@@ -2,7 +2,7 @@ import path from 'node:path';
 import fs from 'node:fs';
 import fsp from 'node:fs/promises';
 
-import { task } from './trace';
+import { SpanCategory, task } from './trace';
 import { treeDir, TreeFileType } from './lib/tree-dir';
 import type { TreeType, TreeTypeArray } from './lib/tree-dir';
 
@@ -54,7 +54,7 @@ async function copyDirContents(srcDir: string, destDir: string, promises: Array<
 }
 
 export const buildPublic = task(require.main === module, __filename)(async (span) => {
-  await span.traceChildAsync('copy rest of the files', async () => {
+  await span.traceChild('copy rest of the files', SpanCategory.FsWrite).traceAsyncFn(async () => {
     const p: Array<Promise<any>> = [];
 
     fs.mkdirSync(OUTPUT_MODULES_DIR, { recursive: true });
@@ -67,7 +67,8 @@ export const buildPublic = task(require.main === module, __filename)(async (span
   });
 
   const html = await span
-    .traceChild('generate index.html')
+    // crawls the whole public dir; the html templating itself is trivial
+    .traceChild('generate index.html', SpanCategory.FsRead)
     .traceAsyncFn(() => treeDir(PUBLIC_DIR).then(generateHtml));
 
   return Promise.all([
diff --git a/Build/build-reject-domainset.ts b/Build/build-reject-domainset.ts
index 75ebb2fc..db46dcfa 100644
--- a/Build/build-reject-domainset.ts
+++ b/Build/build-reject-domainset.ts
@@ -8,7 +8,7 @@ import { processFilterRulesWithPreload } from './lib/parse-filter/filters';
 
 import { HOSTS, ADGUARD_FILTERS, PREDEFINED_WHITELIST, DOMAIN_LISTS, HOSTS_EXTRA, DOMAIN_LISTS_EXTRA, ADGUARD_FILTERS_EXTRA, ADGUARD_FILTERS_WHITELIST, PHISHING_HOSTS_EXTRA, PHISHING_DOMAIN_LISTS_EXTRA, BOGUS_NXDOMAIN_DNSMASQ, ENFORCED_BLACKLIST_FROM_WHITELIST } from './constants/reject-data-source';
 import { readFileIntoProcessedArray } from './lib/fetch-text-by-line';
-import { task } from './trace';
+import { SpanCategory, task } from './trace';
 // tldts-experimental is way faster than tldts, but very little bit inaccurate
 // (since it is hashes based). But the result is still deterministic, which is
 // enough when creating a simple stat of reject hosts.
@@ -220,7 +220,8 @@ export const buildRejectDomainSet = task(require.main === module, __filename)(as
             }
           }
           // return arr;
-        })
+        }),
+        SpanCategory.Network
       ));
 
       return Promise.all(promises);
@@ -244,7 +245,7 @@ export const buildRejectDomainSet = task(require.main === module, __filename)(as
   });
 
   // whitelist
-  span.traceChildSync('whitelist', () => {
+  span.traceChild('whitelist', SpanCategory.Compute).traceSyncFn(() => {
     for (const domain of filterRuleWhitelistDomainSets) {
       rejectDomainsetOutput.whitelistDomain(domain);
       rejectExtraDomainsetOutput.whitelistDomain(domain);
@@ -273,12 +274,13 @@ export const buildRejectDomainSet = task(require.main === module, __filename)(as
     });
   });
 
+  // each write() opens its own RuleOutput#<id> span under the task span
   await Promise.all([
-    span.traceChildAsync('write reject domainset', () => rejectDomainsetOutput.write()),
-    span.traceChildAsync('write reject_extra domainset', () => rejectExtraDomainsetOutput.write()),
-    span.traceChildAsync('write reject_phishing domainset', () => rejectPhisingDomainsetOutput.write()),
-    span.traceChildAsync('write reject ip list', () => rejectIPOutput.write()),
-    span.traceChildAsync('write reject non-ip ruleset', () => rejectNonIpRulesetOutput.write())
+    rejectDomainsetOutput.write(),
+    rejectExtraDomainsetOutput.write(),
+    rejectPhisingDomainsetOutput.write(),
+    rejectIPOutput.write(),
+    rejectNonIpRulesetOutput.write()
   ]);
 
   // we are going to re-use rejectOutput's domainTrie and mutate it
diff --git a/Build/build-speedtest-domainset.ts b/Build/build-speedtest-domainset.ts
index 1d131641..bc319638 100644
--- a/Build/build-speedtest-domainset.ts
+++ b/Build/build-speedtest-domainset.ts
@@ -1,7 +1,7 @@
 import path from 'node:path';
 
 import tldts from 'tldts-experimental';
-import { task } from './trace';
+import { SpanCategory, task } from './trace';
 import { SHARED_DESCRIPTION } from './constants/description';
 import { readFileIntoProcessedArray } from './lib/fetch-text-by-line';
 
@@ -79,7 +79,7 @@ export const buildSpeedtestDomainSet = task(require.main === module, __filename)
     )
     .addFromDomainset(readFileIntoProcessedArray(path.resolve(SOURCE_DIR, 'domainset/speedtest.conf')))
     .addFromDomainset(readFileIntoProcessedArray(path.resolve(OUTPUT_SURGE_DIR, 'domainset/speedtest.conf')))
-    .bulkAddDomain(await span.traceChildPromise('get speedtest.net servers', getSpeedtestHostsGroupsPromise))
-    .bulkAddDomain(await span.traceChildPromise('get librespeed backends', getLibrespeedBackendsPromise))
+    .bulkAddDomain(await span.traceChildPromise('get speedtest.net servers', getSpeedtestHostsGroupsPromise, SpanCategory.Network))
+    .bulkAddDomain(await span.traceChildPromise('get librespeed backends', getLibrespeedBackendsPromise, SpanCategory.Network))
     .write()
 );
diff --git a/Build/build-telegram.worker.ts b/Build/build-telegram.worker.ts
index e7d8100d..05ee139c 100644
--- a/Build/build-telegram.worker.ts
+++ b/Build/build-telegram.worker.ts
@@ -1,5 +1,5 @@
 // @ts-check
-import { task } from './trace';
+import { SpanCategory, task } from './trace';
 import { SHARED_DESCRIPTION } from './constants/description';
 import { RulesetOutput } from './lib/rules/ruleset';
 import { $$fetch } from './lib/fetch-retry';
@@ -16,7 +16,7 @@ const buildTelegramCIDR = task(require.main === module, __filename)(async (span)
     ];
     const ipcidr6: string[] = [];
 
-    const date = await childSpan.traceChildAsync('fetch from official cidr list', async () => {
+    const date = await childSpan.traceChild('fetch from official cidr list', SpanCategory.Network).traceAsyncFn(async () => {
       const resp = await $$fetch('https://core.telegram.org/resources/cidr.txt');
       const lastModified = resp.headers.get('last-modified');
 
@@ -148,7 +148,8 @@ async function fetchConfigFromBootstrapEndpoints() {
 export const buildMTProtoDCConfig = task(require.main === module, __filename)(async (span) => {
   const config = await span.traceChildAsync(
     'fetch help.getConfig',
-    fetchConfigFromBootstrapEndpoints
+    fetchConfigFromBootstrapEndpoints,
+    SpanCategory.Network
   );
 
   const backupEndpoints = await span.traceChildAsync(
diff --git a/Build/download-mock-assets.worker.ts b/Build/download-mock-assets.worker.ts
index e6f1c849..316d3c86 100644
--- a/Build/download-mock-assets.worker.ts
+++ b/Build/download-mock-assets.worker.ts
@@ -1,4 +1,4 @@
-import { task } from './trace';
+import { SpanCategory, task } from './trace';
 import path from 'node:path';
 import fs from 'node:fs';
 import { pipeline } from 'node:stream/promises';
@@ -22,7 +22,9 @@ export const downloadMockAssets = task(require.main === module, __filename)(asyn
 
   return Promise.all(Object.entries(ASSETS_LIST).map(
     ([filename, url]) => span
-      .traceChildAsync(url, async () => {
+      // fetch + stream straight to disk; the transfer dominates
+      .traceChild(url, SpanCategory.Network)
+      .traceAsyncFn(async () => {
         const res = await $$fetch(url);
         if (!res.ok) {
           console.error(`Failed to download ${url}`);
diff --git a/Build/download-previous-build.ts b/Build/download-previous-build.ts
index f461d997..d812be86 100644
--- a/Build/download-previous-build.ts
+++ b/Build/download-previous-build.ts
@@ -1,7 +1,7 @@
 import path from 'node:path';
 import fs from 'node:fs';
 import { pipeline } from 'node:stream/promises';
-import { task } from './trace';
+import { SpanCategory, task } from './trace';
 import { extract as tarExtract } from 'tar-fs';
 import type { Headers as TarEntryHeaders } from 'tar-fs';
 import zlib from 'node:zlib';
@@ -34,9 +34,10 @@ export const downloadPreviousBuild = task(require.main === module, __filename)(a
       return GITLAB_CODELOAD_URL;
     }
     return GITHUB_CODELOAD_URL;
-  });
+  }, SpanCategory.Network);
 
-  return span.traceChildAsync('download & extract previoud build', async () => {
+  // streaming download -> gunzip -> untar to disk, network dominates so that is the tag
+  return span.traceChildAsync('download & extract previous build', async () => {
     const respBody = undici.pipeline(
       tarGzUrl,
       {
@@ -89,5 +90,5 @@ export const downloadPreviousBuild = task(require.main === module, __filename)(a
         }
       )
     );
-  });
+  }, SpanCategory.Network);
 });
diff --git a/Build/index.ts b/Build/index.ts
index 62e59a4b..105b53c1 100644
--- a/Build/index.ts
+++ b/Build/index.ts
@@ -22,8 +22,9 @@ import { createWorker } from './lib/worker';
 import { buildPublic } from './build-public';
 import { buildCloudMounterRules } from './build-cloudmounter-rules';
 
-import { printStats, printTraceResult, whyIsNodeRunning } from './trace';
+import { printBuildReport, whyIsNodeRunning } from './trace';
 import type { TraceResult } from './trace';
+import { performance } from 'node:perf_hooks';
 import { buildDeprecateFiles } from './build-deprecate-files';
 import path from 'node:path';
 import { ROOT_DIR } from './constants/dir';
@@ -71,6 +72,12 @@ const buildFinishedLock = path.join(ROOT_DIR, '.BUILD_FINISHED');
     fs.unlinkSync(buildFinishedLock);
   }
 
+  // Build-wide resource baselines: how busy the main thread's event loop was
+  // (CPU vs. waiting on I/O) and how much CPU the whole process (incl. worker
+  // threads) burned relative to wall-clock.
+  const eluAtStart = performance.eventLoopUtilization();
+  const cpuAtStart = process.cpuUsage();
+
   const microsoftCdnWorker = createWorker<typeof import('./build-microsoft-cdn.worker')>(
     require.resolve('./build-microsoft-cdn.worker')
   )(['buildMicrosoftCdn']);
@@ -130,10 +137,10 @@ const buildFinishedLock = path.join(ROOT_DIR, '.BUILD_FINISHED');
     fs.writeFileSync(buildFinishedLock, 'BUILD_FINISHED\n');
 
     printExternalDownloadStats();
-    traces.forEach((t) => {
-      printTraceResult(t);
+    printBuildReport(traces, {
+      elu: performance.eventLoopUtilization(eluAtStart),
+      cpu: process.cpuUsage(cpuAtStart)
     });
-    printStats(traces);
 
     await Promise.all([
       microsoftCdnWorker.end(),
diff --git a/Build/lib/create-file.ts b/Build/lib/create-file.ts
index 16c4d186..2d86b744 100644
--- a/Build/lib/create-file.ts
+++ b/Build/lib/create-file.ts
@@ -2,6 +2,7 @@ import { fastStringArrayJoin } from 'foxts/fast-string-array-join';
 import fs from 'node:fs';
 import path from 'node:path';
 import picocolors from 'picocolors';
+import { SpanCategory } from '../trace';
 import type { Span } from '../trace';
 import { readFileByLine } from './fetch-text-by-line';
 import { writeFile } from './misc';
@@ -37,7 +38,7 @@ async function isPreviousOutputEqual(span: Span, linesA: string[], filePath: str
     }
 
     return fileEqual(linesA, readFileByLine(filePath));
-  });
+  }, SpanCategory.FsRead);
 
   if (isEqual) {
     console.log(picocolors.gray(picocolors.dim(`same content, bail out writing: ${filePath}`)));
@@ -81,7 +82,8 @@ export async function compareAndWriteFileInWorker(span: Span, linesA: string[],
 export function writeFileLines(span: Span, linesA: string[], filePath: string): Promise<void> {
   return span.traceChildAsync<void>(
     `writing ${filePath}`,
-    () => writeFile(filePath, fastStringArrayJoin(linesA, '\n') + '\n')
+    () => writeFile(filePath, fastStringArrayJoin(linesA, '\n') + '\n'),
+    SpanCategory.FsWrite
   );
 }
 
@@ -89,5 +91,5 @@ export function writeFileLinesSync(span: Span, linesA: string[], filePath: strin
   span.traceChildSync(`writing ${filePath}`, () => {
     fs.mkdirSync(path.dirname(filePath), { recursive: true });
     fs.writeFileSync(filePath, fastStringArrayJoin(linesA, '\n') + '\n');
-  });
+  }, SpanCategory.FsWrite);
 }
diff --git a/Build/lib/fetch-telegram-backup-endpoints.ts b/Build/lib/fetch-telegram-backup-endpoints.ts
index 771db609..aa68ab5f 100644
--- a/Build/lib/fetch-telegram-backup-endpoints.ts
+++ b/Build/lib/fetch-telegram-backup-endpoints.ts
@@ -5,6 +5,7 @@ import picocolors from 'picocolors';
 import { fastStringArrayJoin } from 'foxts/fast-string-array-join';
 import { stableHash } from 'stable-hash';
 
+import { SpanCategory } from '../trace';
 import type { Span } from '../trace';
 import { $$fetch } from './fetch-retry';
 import { getTelegramBackupIPFromBase64 } from './get-telegram-backup-ip';
@@ -37,7 +38,7 @@ async function fetchDnsEndpoints(span: Span, domain: string, traceName: string)
     return Object.assign(resolver, { server: ip });
   });
 
-  await span.traceChildAsync(traceName, () => Promise.all(resolvers.map(async (resolver) => {
+  await span.traceChild(traceName, SpanCategory.Network).traceAsyncFn(() => Promise.all(resolvers.map(async (resolver) => {
     try {
       const response = await resolver.resolveTxt(domain);
       const strings = response.map(result => fastStringArrayJoin(result, ''));
@@ -61,7 +62,7 @@ async function fetchDnsEndpoints(span: Span, domain: string, traceName: string)
 }
 
 async function fetchRealtimeDatabaseEndpoints(span: Span) {
-  return span.traceChildAsync('backup source 2: Firebase Realtime DB', async () => {
+  return span.traceChild('backup source 2: Firebase Realtime DB', SpanCategory.Network).traceAsyncFn(async () => {
     try {
       const data = await (await $$fetch('https://reserve-5a846.firebaseio.com/ipconfigv3.json')).json();
       if (typeof data !== 'string' || data.length !== 344) {
@@ -79,7 +80,7 @@ async function fetchRealtimeDatabaseEndpoints(span: Span) {
 }
 
 async function fetchValueStoreEndpoints(span: Span) {
-  return span.traceChildAsync('backup source 3: Firebase Value Store', async () => {
+  return span.traceChild('backup source 3: Firebase Value Store', SpanCategory.Network).traceAsyncFn(async () => {
     try {
       const json = await (await $$fetch('https://firestore.googleapis.com/v1/projects/reserve-5a846/databases/(default)/documents/ipconfig/v3', {
         headers: {
@@ -110,7 +111,7 @@ async function fetchValueStoreEndpoints(span: Span) {
 }
 
 async function fetchAppEngineEndpoints(span: Span, url: string, traceName: string) {
-  return span.traceChildAsync(traceName, async () => {
+  return span.traceChild(traceName, SpanCategory.Network).traceAsyncFn(async () => {
     try {
       const data = (await (await $$fetch(url)).text()).trim();
       if (data.length !== 344) {
@@ -147,7 +148,7 @@ async function fetchTestEndpoints(span: Span) {
 
 function getProductionEndpoints(span: Span) {
   if (productionEndpointsPromise) {
-    return span.traceChildAsync('reuse production backup endpoints', () => productionEndpointsPromise!);
+    return span.traceChildAsync('reuse production backup endpoints', () => productionEndpointsPromise!, SpanCategory.Wait);
   }
   productionEndpointsPromise = fetchProductionEndpoints(span);
   return productionEndpointsPromise;
@@ -155,7 +156,7 @@ function getProductionEndpoints(span: Span) {
 
 function getTestEndpoints(span: Span) {
   if (testEndpointsPromise) {
-    return span.traceChildAsync('reuse test backup endpoints', () => testEndpointsPromise!);
+    return span.traceChildAsync('reuse test backup endpoints', () => testEndpointsPromise!, SpanCategory.Wait);
   }
   testEndpointsPromise = fetchTestEndpoints(span);
   return testEndpointsPromise;
diff --git a/Build/lib/get-phishing-domains.ts b/Build/lib/get-phishing-domains.ts
index 0decd06c..4ccf5dae 100644
--- a/Build/lib/get-phishing-domains.ts
+++ b/Build/lib/get-phishing-domains.ts
@@ -2,7 +2,7 @@ import picocolors from 'picocolors';
 import { parse } from 'tldts-experimental';
 import { appendArrayInPlaceCurried } from 'foxts/append-array-in-place';
 
-import { workerJob } from '../trace';
+import { SpanCategory, workerJob } from '../trace';
 import type { RawSpan, WorkerJobResult } from '../trace';
 import type { TldTsParsed } from './normalize-domain';
 
@@ -25,7 +25,7 @@ export function getPhishingDomains(rawSpan?: RawSpan, isDebug = false): Promise<
 
     const domainGroups = await Promise.all(downloads.map(task => task(childSpan)));
 
-    return childSpan.traceChildSync<string[]>('calculate and handling mass phishing domains', () => {
+    return childSpan.traceChild('calculate and handling mass phishing domains', SpanCategory.Compute).traceSyncFn<string[]>(() => {
       const domainArr: string[] = [];
 
       domainGroups.forEach(appendArrayInPlaceCurried(domainArr));
diff --git a/Build/lib/parse-filter/domainlists.ts b/Build/lib/parse-filter/domainlists.ts
index f2b34430..67501262 100644
--- a/Build/lib/parse-filter/domainlists.ts
+++ b/Build/lib/parse-filter/domainlists.ts
@@ -1,6 +1,7 @@
 import { fastNormalizeDomain, fastNormalizeDomainWithoutWww } from '../normalize-domain';
 import { onBlackFound } from './shared';
 import { fetchAssets } from '../fetch-assets';
+import { SpanCategory } from '../../trace';
 import type { Span } from '../../trace';
 
 function domainListLineCb(line: string, set: string[], meta: string, normalizeDomain = fastNormalizeDomain) {
@@ -29,14 +30,14 @@ export function processDomainListsWithPreload(
   const lineCb = includeAllSubDomain ? domainListLineCbIncludeAllSubdomain : domainListLineCb;
 
   return (span: Span) => span.traceChildAsync(`process domainlist: ${domainListsUrl}`, async (childSpan) => {
-    const filterRules = await childSpan.traceChildPromise('download', downloadPromise);
+    const filterRules = await childSpan.traceChildPromise('download', downloadPromise, SpanCategory.Network);
     const domainSets: string[] = [];
 
     childSpan.traceChildSync('parse domain list', () => {
       for (let i = 0, len = filterRules.length; i < len; i++) {
         lineCb(filterRules[i], domainSets, domainListsUrl, fastNormalizeDomainWithoutWww);
       }
-    });
+    }, SpanCategory.Compute);
 
     return domainSets;
   });
diff --git a/Build/lib/parse-filter/filters.ts b/Build/lib/parse-filter/filters.ts
index 659f14b7..0262a3f4 100644
--- a/Build/lib/parse-filter/filters.ts
+++ b/Build/lib/parse-filter/filters.ts
@@ -1,4 +1,5 @@
 import picocolors from 'picocolors';
+import { SpanCategory } from '../../trace';
 import type { Span } from '../../trace';
 import { fetchAssets } from '../fetch-assets';
 import { onBlackFound, onWhiteFound } from './shared';
@@ -49,7 +50,7 @@ export function processFilterRulesWithPreload(
       filterRulesUrl: string
     }
   >(`process filter rules: ${filterRulesUrl}`, async (span) => {
-    const filterRules = await span.traceChildPromise('download', downloadPromise);
+    const filterRules = await span.traceChildPromise('download', downloadPromise, SpanCategory.Network);
 
     const whiteDomains = new Set<string>();
     const whiteDomainSuffixes = new Set<string>();
@@ -121,7 +122,7 @@ export function processFilterRulesWithPreload(
       }
     };
 
-    span.traceChild('parse adguard filter').traceSyncFn(() => {
+    span.traceChild('parse adguard filter', SpanCategory.Compute).traceSyncFn(() => {
       for (let i = 0, len = filterRules.length; i < len; i++) {
         lineCb(filterRules[i]);
       }
diff --git a/Build/lib/parse-filter/hosts.ts b/Build/lib/parse-filter/hosts.ts
index 05d38d65..dbfc7ad3 100644
--- a/Build/lib/parse-filter/hosts.ts
+++ b/Build/lib/parse-filter/hosts.ts
@@ -1,3 +1,4 @@
+import { SpanCategory } from '../../trace';
 import type { Span } from '../../trace';
 import { fetchAssets } from '../fetch-assets';
 import { fastNormalizeDomainWithoutWww } from '../normalize-domain';
@@ -42,11 +43,11 @@ export function processHosts(
   const cb = includeAllSubDomain ? hostsLineCbIncludeAllSubdomain : hostsLineCb;
 
   return span.traceChildAsync(`process hosts: ${hostsUrl}`, async (span) => {
-    const filterRules = await span.traceChild('download').traceAsyncFn(() => fetchAssets(hostsUrl, mirrors, true));
+    const filterRules = await span.traceChild('download', SpanCategory.Network).traceAsyncFn(() => fetchAssets(hostsUrl, mirrors, true));
 
     const domainSets: string[] = [];
 
-    span.traceChild('parse hosts').traceSyncFn(() => {
+    span.traceChild('parse hosts', SpanCategory.Compute).traceSyncFn(() => {
       for (let i = 0, len = filterRules.length; i < len; i++) {
         cb(filterRules[i], domainSets, hostsUrl);
       }
@@ -61,11 +62,11 @@ export function processHostsWithPreload(hostsUrl: string, mirrors: string[] | nu
   const cb = includeAllSubDomain ? hostsLineCbIncludeAllSubdomain : hostsLineCb;
 
   return (span: Span) => span.traceChildAsync(`process hosts: ${hostsUrl}`, async (span) => {
-    const filterRules = await span.traceChild('download').tracePromise(downloadPromise);
+    const filterRules = await span.traceChild('download', SpanCategory.Network).tracePromise(downloadPromise);
 
     const domainSets: string[] = [];
 
-    span.traceChild('parse hosts').traceSyncFn(() => {
+    span.traceChild('parse hosts', SpanCategory.Compute).traceSyncFn(() => {
       for (let i = 0, len = filterRules.length; i < len; i++) {
         cb(filterRules[i], domainSets, hostsUrl);
       }
diff --git a/Build/lib/rules/base.ts b/Build/lib/rules/base.ts
index 1c8b2222..95c5cf91 100644
--- a/Build/lib/rules/base.ts
+++ b/Build/lib/rules/base.ts
@@ -1,3 +1,4 @@
+import { SpanCategory } from '../../trace';
 import type { Span } from '../../trace';
 import { HostnameSmolTrie } from 'hntrie/smol';
 import { not, nullthrow } from 'foxts/guard';
@@ -68,10 +69,15 @@ export class FileOutput {
     return this;
   };
 
-  protected readonly span: Span;
+  /**
+   * The `RuleOutput#id` span is only opened by write(): between construction and
+   * write() this object merely accumulates sources, and that time already belongs
+   * to the sibling spans doing the downloading / reading.
+   */
+  protected readonly parentSpan: Span;
 
   constructor($span: Span, protected readonly id: string) {
-    this.span = $span.traceChild('RuleOutput#' + id);
+    this.parentSpan = $span;
   }
 
   protected title: string | null = null;
@@ -463,10 +469,12 @@ export class FileOutput {
   }
 
   write(): Promise<unknown> {
-    return this.span.traceChildAsync('write all', async (childSpan) => {
-      await childSpan.traceChildAsync('done', () => this.done());
+    return this.parentSpan.traceChildAsync('RuleOutput#' + this.id, async (childSpan) => {
+      // pendingPromise is the (untraced) reading + parsing of every source added
+      // via addFromRuleset / addFromDomainset, so this is waiting on fs + compute
+      await childSpan.traceChildAsync('done', () => this.done(), SpanCategory.Wait);
 
-      const domains = childSpan.traceChildSync('dump domain trie', () => this.dumpDomains());
+      const domains = childSpan.traceChildSync('dump domain trie', () => this.dumpDomains(), SpanCategory.Compute);
 
       const title = nullthrow(this.title, 'Missing title');
       const descriptions = nullthrow(this.description, 'Missing description');
@@ -505,7 +513,7 @@ export class FileOutput {
         );
       }
 
-      childSpan.traceChildSync('write to strategies', () => this.writeToStrategies(domains));
+      childSpan.traceChildSync('write to strategies', () => this.writeToStrategies(domains), SpanCategory.Compute);
 
       return childSpan.traceChildAsync('output to disk', (childSpan) => {
         const promises: Array<Promise<void>> = [];
@@ -521,7 +529,8 @@ export class FileOutput {
               isMainThread
                 ? strategy.output(childSpan, title, descriptions, this.date, filePath)
                 : strategy.outputInWorker(childSpan, title, descriptions, this.date, filePath)
-            ))
+            // self time here is banner + content hash; compare / writing are traced as children
+            ), SpanCategory.Compute)
           );
         }
 
diff --git a/Build/lib/rules/output.worker.ts b/Build/lib/rules/output.worker.ts
index bc0aa147..af6b4e2d 100644
--- a/Build/lib/rules/output.worker.ts
+++ b/Build/lib/rules/output.worker.ts
@@ -1,4 +1,4 @@
-import { workerJob } from '../../trace';
+import { SpanCategory, workerJob } from '../../trace';
 import type { RawSpan, WorkerJobResult } from '../../trace';
 import { resolveStrategyOutputPath, reviveStrategy, writeDataToStrategies } from './strategy-write-data';
 import type { OutputWorkerPayload } from './strategy-write-data';
@@ -15,7 +15,7 @@ export function writeOutput(rawSpan: RawSpan | undefined, payload: OutputWorkerP
   return workerJob(rawSpan, (span) => {
     const strategies = payload.strategies.map(reviveStrategy);
 
-    span.traceChildSync('write to strategies', () => writeDataToStrategies(payload.data, strategies));
+    span.traceChildSync('write to strategies', () => writeDataToStrategies(payload.data, strategies), SpanCategory.Compute);
 
     const date = new Date(payload.dateMs);
 
@@ -29,7 +29,9 @@ export function writeOutput(rawSpan: RawSpan | undefined, payload: OutputWorkerP
         // eslint-disable-next-line no-await-in-loop -- see above
         await childSpan.traceChildAsync(
           'write ' + strategy.name,
-          (strategySpan) => strategy.outputInWorker(strategySpan, payload.title, payload.description, date, filePath)
+          (strategySpan) => strategy.outputInWorker(strategySpan, payload.title, payload.description, date, filePath),
+          // self time here is banner + content hash; compare / writing are traced as children
+          SpanCategory.Compute
         );
       }
     });
diff --git a/Build/tools-dedupe-src.ts b/Build/tools-dedupe-src.ts
index 542b633a..7ddb3cae 100644
--- a/Build/tools-dedupe-src.ts
+++ b/Build/tools-dedupe-src.ts
@@ -5,7 +5,7 @@ import { SOURCE_DIR } from './constants/dir';
 import { readFileByLine } from './lib/fetch-text-by-line';
 import { processLine } from './lib/process-line';
 import { HostnameSmolTrie } from 'hntrie/smol';
-import { task } from './trace';
+import { SpanCategory, task } from './trace';
 import { fastStringArrayJoin } from 'foxts/fast-string-array-join';
 
 const ENFORCED_WHITELIST = [
@@ -26,7 +26,7 @@ const ENFORCED_WHITELIST = [
 const DEDUPE_LIST: string[] = ['adx-static.ksosoft.com', 'dns.iqiyi.com', 'domain.expiring-soon.xyz', 'img.catwvod.xyz', 'img.vim-cn.com', 's3-zen.mds.yandex.net'];
 
 task(require.main === module, __filename)(async (span) => {
-  const files = await span.traceChildAsync('crawl thru all files', () => new Fdir()
+  const files = await span.traceChild('crawl thru all files', SpanCategory.FsRead).traceAsyncFn(() => new Fdir()
     .withFullPaths()
     .filter((filepath, isDirectory) => {
       if (isDirectory) return true;
@@ -38,7 +38,7 @@ task(require.main === module, __filename)(async (span) => {
     .crawl(SOURCE_DIR)
     .withPromise());
 
-  const whiteTrie = span.traceChildSync('build whitelist trie', () => {
+  const whiteTrie = span.traceChild('build whitelist trie', SpanCategory.Compute).traceSyncFn(() => {
     const trie = new HostnameSmolTrie(DEDUPE_LIST);
     ENFORCED_WHITELIST.forEach((item) => trie.whitelist(item));
     return trie;
diff --git a/Build/trace/index.ts b/Build/trace/index.ts
index c2cf1151..03909daa 100644
--- a/Build/trace/index.ts
+++ b/Build/trace/index.ts
@@ -1,40 +1,36 @@
 import { isCI } from 'ci-info';
 import { noop } from 'foxts/noop';
 import { basename, extname } from 'node:path';
+import { performance } from 'node:perf_hooks';
 import process from 'node:process';
-import picocolors from 'picocolors';
+import { threadId } from 'node:worker_threads';
 import { mergeExternalDownloadStats, takeExternalDownloadStats } from '../lib/download-stats';
 import type { ExternalDownloadStatsSnapshot } from '../lib/download-stats';
+import { SPAN_STATUS_END, SPAN_STATUS_START, SpanCategory } from './types';
+import type { RawSpan, TraceResult } from './types';
+import { adjustTraceTimestamps, printBuildReport } from './report';
 
-export const SPAN_STATUS_START = 0;
-export const SPAN_STATUS_END = 1;
+export { SPAN_STATUS_START, SPAN_STATUS_END, SpanCategory, UNCATEGORIZED } from './types';
+export type { RawSpan, TraceResult, ReportedSpanCategory, SpanEventLoopUtilization } from './types';
+export { printTraceResult, printStats, printBuildReport, analyzeTraces } from './report';
+export type { BuildResourceUsage, TraceAnalysis } from './report';
 
 const spanTag = Symbol('span');
 
-export interface TraceResult {
-  name: string,
-  start: number,
-  end: number,
-  children: TraceResult[]
-}
-
-/** Pure data object — safe to transfer across Worker Thread boundaries. */
-export interface RawSpan {
-  traceResult: TraceResult,
-  status: typeof SPAN_STATUS_START | typeof SPAN_STATUS_END
-}
-
 export interface Span {
   [spanTag]: true,
   readonly rawSpan: RawSpan,
   readonly stop: (time?: number) => void,
-  readonly traceChild: (name: string) => Span,
+  /** Tag (or re-tag) what this span's self time is spent on */
+  readonly setCategory: (category: SpanCategory) => Span,
+  readonly traceChild: (name: string, category?: SpanCategory) => Span,
   readonly traceSyncFn: <T>(fn: (span: Span) => T) => T,
   readonly traceAsyncFn: <T>(fn: (span: Span) => T | Promise<T>) => Promise<T>,
   readonly tracePromise: <T>(promise: Promise<T>) => Promise<T>,
-  readonly traceChildSync: <T>(name: string, fn: (span: Span) => T) => T,
-  readonly traceChildAsync: <T>(name: string, fn: (span: Span) => Promise<T>) => Promise<T>,
-  readonly traceChildPromise: <T>(name: string, promise: Promise<T>) => Promise<T>,
+  readonly traceChildSync: <T>(name: string, fn: (span: Span) => T, category?: SpanCategory) => T,
+  readonly traceChildAsync: <T>(name: string, fn: (span: Span) => T | Promise<T>, category?: SpanCategory) => Promise<T>,
+  readonly traceChildPromise: <T>(name: string, promise: Promise<T>, category?: SpanCategory) => Promise<T>,
+  /** Always tagged {@link SpanCategory.Worker}: the self time is the IPC + waiting on the worker */
   readonly traceWorkerChild: <T>(name: string, factory: (rawSpan: RawSpan) => Promise<WorkerJobResult<T>>) => Promise<T>,
   readonly traceResult: TraceResult
 }
@@ -52,17 +48,26 @@ export function makeSpan(rawSpan: RawSpan): Span {
       throw new Error(`span already stopped: ${traceResult.name}`);
     }
     traceResult.end = time ?? performance.now();
+    if (rawSpan.eluStart) {
+      const elu = performance.eventLoopUtilization(rawSpan.eluStart);
+      traceResult.elu = { idle: elu.idle, active: elu.active };
+    }
     rawSpan.status = SPAN_STATUS_END;
   };
 
-  const traceChild = (name: string) => createSpan(name, traceResult);
+  const traceChild = (name: string, category?: SpanCategory) => createSpan(name, traceResult, category);
 
   const span: Span = {
     [spanTag]: true,
     rawSpan,
     stop,
+    setCategory(category) {
+      traceResult.category = category;
+      return span;
+    },
     traceChild,
     traceSyncFn<T>(fn: (span: Span) => T) {
+      traceResult.sync = true;
       const res = fn(span);
       span.stop();
       return res;
@@ -78,12 +83,12 @@ export function makeSpan(rawSpan: RawSpan): Span {
       span.stop();
       return res;
     },
-    traceChildSync: <T>(name: string, fn: (span: Span) => T): T => traceChild(name).traceSyncFn(fn),
-    traceChildAsync: <T>(name: string, fn: (span: Span) => T | Promise<T>): Promise<T> => traceChild(name).traceAsyncFn(fn),
-    traceChildPromise: <T>(name: string, promise: Promise<T>): Promise<T> => traceChild(name).tracePromise(promise),
+    traceChildSync: <T>(name: string, fn: (span: Span) => T, category?: SpanCategory): T => traceChild(name, category).traceSyncFn(fn),
+    traceChildAsync: <T>(name: string, fn: (span: Span) => T | Promise<T>, category?: SpanCategory): Promise<T> => traceChild(name, category).traceAsyncFn(fn),
+    traceChildPromise: <T>(name: string, promise: Promise<T>, category?: SpanCategory): Promise<T> => traceChild(name, category).tracePromise(promise),
 
     async traceWorkerChild<T>(name: string, factory: (rawSpan: RawSpan) => Promise<WorkerJobResult<T>>): Promise<T> {
-      const childSpan = traceChild(name);
+      const childSpan = traceChild(name, SpanCategory.Worker);
       const { result, traceResult, workerTimeOrigin, externalDownloadStats } = await factory(childSpan.rawSpan);
       mergeWorkerTrace(childSpan, traceResult, workerTimeOrigin);
       mergeExternalDownloadStats(externalDownloadStats);
@@ -96,18 +101,25 @@ export function makeSpan(rawSpan: RawSpan): Span {
   return span;
 }
 
-export function createSpan(name: string, parentTraceResult?: TraceResult): Span {
+export function createSpan(name: string, parentTraceResult?: TraceResult, category?: SpanCategory): Span {
+  const traceResult: TraceResult = {
+    name,
+    start: performance.now(),
+    end: 0,
+    thread: threadId,
+    children: []
+  };
+  if (category !== undefined) {
+    traceResult.category = category;
+  }
+
   const rawSpan: RawSpan = {
-    traceResult: {
-      name,
-      start: performance.now(),
-      end: 0,
-      children: []
-    },
-    status: SPAN_STATUS_START
+    traceResult,
+    status: SPAN_STATUS_START,
+    eluStart: performance.eventLoopUtilization()
   };
 
-  parentTraceResult?.children.push(rawSpan.traceResult);
+  parentTraceResult?.children.push(traceResult);
 
   return makeSpan(rawSpan);
 }
@@ -123,6 +135,8 @@ export function task(importMetaMain: boolean, importMetaPath: string) {
     };
 
     if (importMetaMain) {
+      const eluAtStart = performance.eventLoopUtilization();
+      const cpuAtStart = process.cpuUsage();
       const innerSpan = createSpan(taskName);
 
       process.on('uncaughtException', (error) => {
@@ -136,7 +150,11 @@ export function task(importMetaMain: boolean, importMetaPath: string) {
 
       innerSpan.traceChildAsync('dummy', (childSpan) => fn(childSpan, onCleanup)).finally(() => {
         innerSpan.stop();
-        printTraceResult(innerSpan.traceResult);
+        innerSpan.traceResult.timeOrigin = performance.timeOrigin;
+        printBuildReport([innerSpan.traceResult], {
+          elu: performance.eventLoopUtilization(eluAtStart),
+          cpu: process.cpuUsage(cpuAtStart)
+        });
         process.nextTick(whyIsNodeRunning);
         process.nextTick(() => process.exit(0));
       });
@@ -157,6 +175,9 @@ export function task(importMetaMain: boolean, importMetaPath: string) {
         cleanup();
       }
 
+      // A task may run on a worker thread (via jest-worker) and hand its trace
+      // back to the main thread, which then needs this to re-align the clock.
+      runSpan.traceResult.timeOrigin = performance.timeOrigin;
       return runSpan.traceResult;
     }
 
@@ -186,15 +207,6 @@ export async function whyIsNodeRunning() {
 //   };
 // };
 
-function adjustTraceTimestamps(trace: TraceResult, offset: number): TraceResult {
-  return {
-    name: trace.name,
-    start: trace.start + offset,
-    end: trace.end + offset,
-    children: trace.children.map(child => adjustTraceTimestamps(child, offset))
-  };
-}
-
 function mergeWorkerTrace(
   parentSpan: Span,
   workerTraceResult: TraceResult,
@@ -240,67 +252,3 @@ export async function workerJob<T>(
     externalDownloadStats: takeExternalDownloadStats()
   };
 }
-
-export function printTraceResult(traceResult: TraceResult) {
-  printTree(
-    traceResult,
-    node => {
-      if (node.end - node.start < 0) {
-        return node.name;
-      }
-      return `${node.name} ${picocolors.bold(`${(node.end - node.start).toFixed(3)}ms`)}`;
-    }
-  );
-}
-
-function printTree(initialTree: TraceResult, printNode: (node: TraceResult, branch: string) => string) {
-  function printBranch(tree: TraceResult, branch: string, isGraphHead: boolean, isChildOfLastBranch: boolean) {
-    const children = tree.children;
-
-    let branchHead = '';
-
-    if (!isGraphHead) {
-      branchHead = children.length > 0 ? '┬ ' : '─ ';
-    }
-
-    const toPrint = printNode(tree, `${branch}${branchHead}`);
-
-    if (typeof toPrint === 'string') {
-      console.log(`${branch}${branchHead}${toPrint}`);
-    }
-
-    let baseBranch = branch;
-
-    if (!isGraphHead) {
-      baseBranch = branch.slice(0, -2) + (isChildOfLastBranch ? '  ' : '│ ');
-    }
-
-    const nextBranch = `${baseBranch}├─`;
-    const lastBranch = `${baseBranch}└─`;
-
-    children.forEach((child, index) => {
-      const last = children.length - 1 === index;
-      printBranch(child, last ? lastBranch : nextBranch, false, last);
-    });
-  }
-
-  printBranch(initialTree, '', true, false);
-}
-
-export function printStats(stats: TraceResult[]): void {
-  const longestTaskName = Math.max(...stats.map(i => i.name.length));
-  const realStart = Math.min(...stats.map(i => i.start));
-  const realEnd = Math.max(...stats.map(i => i.end));
-
-  const statsStep = ((realEnd - realStart) / 120) | 0;
-
-  stats
-    .sort((a, b) => a.start - b.start)
-    .forEach(stat => {
-      console.log(
-        `[${stat.name}]${' '.repeat(longestTaskName - stat.name.length)}`,
-        ' '.repeat(((stat.start - realStart) / statsStep) | 0),
-        '='.repeat(Math.max(((stat.end - stat.start) / statsStep) | 0, 1))
-      );
-    });
-}
diff --git a/Build/trace/report.test.ts b/Build/trace/report.test.ts
new file mode 100644
index 00000000..c3e4da61
--- /dev/null
+++ b/Build/trace/report.test.ts
@@ -0,0 +1,102 @@
+import { describe, it } from 'mocha';
+import { expect } from 'earl';
+import { performance } from 'node:perf_hooks';
+
+import { analyzeTraces } from './report';
+import { SpanCategory, UNCATEGORIZED } from './types';
+import type { TraceResult } from './types';
+import type { CategoryStat, TraceAnalysis } from './report';
+
+function span(
+  name: string,
+  start: number,
+  end: number,
+  extra: Partial<Pick<TraceResult, 'category' | 'thread' | 'sync' | 'timeOrigin'>> = {},
+  children: TraceResult[] = []
+): TraceResult {
+  return { name, start, end, thread: 0, children, ...extra };
+}
+
+function indexByCategory(analysis: TraceAnalysis) {
+  return analysis.categories.reduce<Record<string, CategoryStat>>((acc, c) => {
+    acc[c.category] = c;
+    return acc;
+  }, {});
+}
+
+describe('trace report analysis', () => {
+  it('attributes self time as duration minus the union of (overlapping) children', () => {
+    const task = span('task', 0, 100, {}, [
+      // two async children that overlap: 10-50 and 30-70 cover 60ms, not 80ms
+      span('a', 10, 50, { category: SpanCategory.Network }),
+      span('b', 30, 70, { category: SpanCategory.Network }),
+      span('c', 80, 90, { category: SpanCategory.Compute, sync: true })
+    ]);
+
+    const analysis = analyzeTraces([task]);
+    const byCategory = indexByCategory(analysis);
+
+    expect(analysis.wall).toEqual(100);
+    expect(byCategory[UNCATEGORIZED].selfTotal).toEqual(100 - 60 - 10);
+    // self of leaves is their full duration, summed across the concurrent pair
+    expect(byCategory[SpanCategory.Network].selfTotal).toEqual(40 + 40);
+    // coverage de-duplicates the overlap
+    expect(byCategory[SpanCategory.Network].coverage).toEqual(60);
+    expect(byCategory[SpanCategory.Compute].selfTotal).toEqual(10);
+    expect(byCategory[SpanCategory.Compute].selfSync).toEqual(10);
+    expect(byCategory[SpanCategory.Network].selfSync).toEqual(0);
+
+    expect(analysis.tasks[0].byCategory[SpanCategory.Network]).toEqual(80);
+    expect(analysis.tasks[0].byCategory[UNCATEGORIZED]).toEqual(30);
+  });
+
+  it('clips children to the parent window and never reports negative self time', () => {
+    // fire-and-forget child that outlives its parent
+    const task = span('task', 0, 50, {}, [span('late', 40, 90, { category: SpanCategory.FsWrite })]);
+
+    const analysis = analyzeTraces([task]);
+    const byCategory = indexByCategory(analysis);
+
+    expect(byCategory[UNCATEGORIZED].selfTotal).toEqual(40);
+    expect(byCategory[SpanCategory.FsWrite].selfTotal).toEqual(50);
+    expect(analysis.wall).toEqual(90);
+  });
+
+  it('skips unfinished spans but counts them', () => {
+    const task = span('task', 0, 10, {}, [span('never stopped', 5, 0)]);
+
+    const analysis = analyzeTraces([task]);
+
+    expect(analysis.unfinishedSpans).toEqual(1);
+    // the unfinished child does not eat into the parent's self time
+    expect(analysis.categories.find(c => c.category === UNCATEGORIZED)!.selfTotal).toEqual(10);
+  });
+
+  it('splits self time between the main thread and workers', () => {
+    const task = span('task', 0, 100, {}, [
+      span('offload', 0, 100, { category: SpanCategory.Worker }, [
+        span('crunch', 20, 60, { category: SpanCategory.Compute, thread: 7, sync: true })
+      ])
+    ]);
+
+    const analysis = analyzeTraces([task]);
+    const compute = analysis.categories.find(c => c.category === SpanCategory.Compute)!;
+    const worker = analysis.categories.find(c => c.category === SpanCategory.Worker)!;
+
+    expect(compute.selfOnWorkers).toEqual(40);
+    expect(compute.selfOnMainThread).toEqual(0);
+    expect(worker.selfOnMainThread).toEqual(60);
+  });
+
+  it('shifts a trace produced on another thread onto the local clock', () => {
+    // A worker whose clock started 1000ms after ours reports everything 1000ms early
+    const workerTask = span('worker task', 0, 10, { timeOrigin: performance.timeOrigin + 1000 });
+    const mainTask = span('main task', 1000, 1010);
+
+    const analysis = analyzeTraces([workerTask, mainTask]);
+
+    expect(analysis.wallStart).toEqual(1000);
+    expect(analysis.wallEnd).toEqual(1010);
+    expect(analysis.tasks[0].node.start).toEqual(1000);
+  });
+});
diff --git a/Build/trace/report.ts b/Build/trace/report.ts
new file mode 100644
index 00000000..64d295ad
--- /dev/null
+++ b/Build/trace/report.ts
@@ -0,0 +1,596 @@
+import { performance } from 'node:perf_hooks';
+import { stripVTControlCharacters } from 'node:util';
+import picocolors from 'picocolors';
+import { SpanCategory, UNCATEGORIZED } from './types';
+import type { ReportedSpanCategory, SpanEventLoopUtilization, TraceResult } from './types';
+
+/** Fixed display order: I/O flavours first, then compute, then the "waiting on something else" buckets */
+const CATEGORY_ORDER: ReportedSpanCategory[] = [
+  SpanCategory.Network,
+  SpanCategory.FsRead,
+  SpanCategory.FsWrite,
+  SpanCategory.Compute,
+  SpanCategory.Worker,
+  SpanCategory.Wait,
+  UNCATEGORIZED
+];
+
+const CATEGORY_COLOR: Record<ReportedSpanCategory, (s: string) => string> = {
+  [SpanCategory.Network]: picocolors.cyan,
+  [SpanCategory.FsRead]: picocolors.green,
+  [SpanCategory.FsWrite]: picocolors.yellow,
+  [SpanCategory.Compute]: picocolors.magenta,
+  [SpanCategory.Worker]: picocolors.blue,
+  [SpanCategory.Wait]: picocolors.gray,
+  [UNCATEGORIZED]: picocolors.gray
+};
+
+const TOP_SPANS_PER_CATEGORY = 5;
+const TOP_SPANS_OVERALL = 15;
+
+// ---------------------------------------------------------------------------
+// Clock alignment
+// ---------------------------------------------------------------------------
+
+export function adjustTraceTimestamps(trace: TraceResult, offset: number): TraceResult {
+  const adjusted: TraceResult = {
+    ...trace,
+    start: trace.start + offset,
+    end: trace.end + offset,
+    children: trace.children.map(child => adjustTraceTimestamps(child, offset))
+  };
+  // the clock is now the main thread's, so the marker no longer applies
+  delete adjusted.timeOrigin;
+  return adjusted;
+}
+
+/**
+ * A task that ran on a worker thread reports `performance.now()` values relative
+ * to that thread's own `timeOrigin`. Shift them onto the current thread's clock
+ * so tasks can be laid out on a shared timeline.
+ */
+export function normalizeTraceClock(trace: TraceResult): TraceResult {
+  if (trace.timeOrigin == null || trace.timeOrigin === performance.timeOrigin) {
+    return trace;
+  }
+  return adjustTraceTimestamps(trace, trace.timeOrigin - performance.timeOrigin);
+}
+
+// ---------------------------------------------------------------------------
+// Analysis
+// ---------------------------------------------------------------------------
+
+export interface AnalyzedSpan {
+  node: TraceResult,
+  /** Ancestor names, root task first, this span last */
+  path: string[],
+  category: ReportedSpanCategory,
+  duration: number,
+  /** Duration not covered by any traced child */
+  self: number
+}
+
+export interface CategoryStat {
+  category: ReportedSpanCategory,
+  /** Sum of self time of all spans with this category (across all threads, concurrency included) */
+  selfTotal: number,
+  /** Wall-clock during which at least one span of this category was in flight */
+  coverage: number,
+  spans: number,
+  selfOnMainThread: number,
+  selfOnWorkers: number,
+  /** Self time of synchronous spans: guaranteed CPU, no event-loop queueing inside */
+  selfSync: number,
+  top: AnalyzedSpan[]
+}
+
+export interface TaskStat {
+  node: TraceResult,
+  duration: number,
+  /** Self time of all descendants (and the task itself) bucketed by category */
+  byCategory: Record<ReportedSpanCategory, number>
+}
+
+export interface TraceAnalysis {
+  wallStart: number,
+  wallEnd: number,
+  wall: number,
+  categories: CategoryStat[],
+  tasks: TaskStat[],
+  topSpans: AnalyzedSpan[],
+  unfinishedSpans: number
+}
+
+type Interval = [start: number, end: number];
+
+function isFinished(node: TraceResult) {
+  return node.end >= node.start;
+}
+
+/** Total length covered by the union of the intervals (handles overlap, which async children routinely do) */
+function unionLength(intervals: Interval[]): number {
+  if (intervals.length === 0) {
+    return 0;
+  }
+  intervals.sort((a, b) => a[0] - b[0]);
+
+  let total = 0;
+  let [curStart, curEnd] = intervals[0];
+  for (let i = 1, len = intervals.length; i < len; i++) {
+    const [start, end] = intervals[i];
+    if (start <= curEnd) {
+      if (end > curEnd) {
+        curEnd = end;
+      }
+    } else {
+      total += curEnd - curStart;
+      curStart = start;
+      curEnd = end;
+    }
+  }
+  return total + (curEnd - curStart);
+}
+
+function selfTime(node: TraceResult): number {
+  const duration = node.end - node.start;
+  if (node.children.length === 0) {
+    return duration;
+  }
+
+  const covered: Interval[] = [];
+  for (let i = 0, len = node.children.length; i < len; i++) {
+    const child = node.children[i];
+    if (!isFinished(child)) {
+      continue;
+    }
+    // clip to the parent's own window: a child stopped after its parent
+    // (fire-and-forget) must not produce negative self time
+    const start = Math.max(child.start, node.start);
+    const end = Math.min(child.end, node.end);
+    if (end > start) {
+      covered.push([start, end]);
+    }
+  }
+
+  return Math.max(0, duration - unionLength(covered));
+}
+
+function emptyByCategory(): Record<ReportedSpanCategory, number> {
+  return {
+    [SpanCategory.Network]: 0,
+    [SpanCategory.FsRead]: 0,
+    [SpanCategory.FsWrite]: 0,
+    [SpanCategory.Compute]: 0,
+    [SpanCategory.Worker]: 0,
+    [SpanCategory.Wait]: 0,
+    [UNCATEGORIZED]: 0
+  };
+}
+
+export function analyzeTraces(rawTraces: TraceResult[]): TraceAnalysis {
+  const traces = rawTraces.map(normalizeTraceClock);
+
+  const spans: AnalyzedSpan[] = [];
+  const coverageIntervals = new Map<ReportedSpanCategory, Interval[]>();
+  const categoryStats = new Map<ReportedSpanCategory, CategoryStat>();
+  for (let i = 0, len = CATEGORY_ORDER.length; i < len; i++) {
+    const category = CATEGORY_ORDER[i];
+    coverageIntervals.set(category, []);
+    categoryStats.set(category, {
+      category,
+      selfTotal: 0,
+      coverage: 0,
+      spans: 0,
+      selfOnMainThread: 0,
+      selfOnWorkers: 0,
+      selfSync: 0,
+      top: []
+    });
+  }
+
+  let unfinishedSpans = 0;
+  let wallStart = Infinity;
+  let wallEnd = -Infinity;
+
+  const walk = (node: TraceResult, path: string[], task: TaskStat) => {
+    const ownPath = path.concat(node.name);
+
+    if (isFinished(node)) {
+      const category = node.category ?? UNCATEGORIZED;
+      const self = selfTime(node);
+      spans.push({ node, path: ownPath, category, duration: node.end - node.start, self });
+
+      const stat = categoryStats.get(category)!;
+      stat.selfTotal += self;
+      stat.spans++;
+      if (node.thread === 0) {
+        stat.selfOnMainThread += self;
+      } else {
+        stat.selfOnWorkers += self;
+      }
+      if (node.sync) {
+        stat.selfSync += self;
+      }
+      coverageIntervals.get(category)!.push([node.start, node.end]);
+      task.byCategory[category] += self;
+
+      if (node.start < wallStart) wallStart = node.start;
+      if (node.end > wallEnd) wallEnd = node.end;
+    } else {
+      unfinishedSpans++;
+    }
+
+    for (let i = 0, len = node.children.length; i < len; i++) {
+      walk(node.children[i], ownPath, task);
+    }
+  };
+
+  const tasks: TaskStat[] = traces.map((trace) => {
+    const task: TaskStat = {
+      node: trace,
+      duration: isFinished(trace) ? trace.end - trace.start : 0,
+      byCategory: emptyByCategory()
+    };
+    walk(trace, [], task);
+    return task;
+  });
+
+  spans.sort((a, b) => b.self - a.self);
+
+  const categories = CATEGORY_ORDER.map((category) => {
+    const stat = categoryStats.get(category)!;
+    stat.coverage = unionLength(coverageIntervals.get(category)!);
+    stat.top = spans.filter(s => s.category === category && s.self > 0).slice(0, TOP_SPANS_PER_CATEGORY);
+    return stat;
+  });
+
+  return {
+    wallStart,
+    wallEnd,
+    wall: wallEnd > wallStart ? wallEnd - wallStart : 0,
+    categories,
+    tasks,
+    topSpans: spans.filter(s => s.self > 0).slice(0, TOP_SPANS_OVERALL),
+    unfinishedSpans
+  };
+}
+
+// ---------------------------------------------------------------------------
+// Formatting helpers
+// ---------------------------------------------------------------------------
+
+function fmtMs(ms: number): string {
+  if (ms >= 10000) {
+    return `${(ms / 1000).toFixed(2)}s`;
+  }
+  if (ms >= 1000) {
+    return `${(ms / 1000).toFixed(3)}s`;
+  }
+  return `${ms.toFixed(1)}ms`;
+}
+
+function fmtPercent(part: number, whole: number): string {
+  if (whole <= 0) {
+    return 'n/a';
+  }
+  return `${(part / whole * 100).toFixed(0)}%`;
+}
+
+function categoryTag(category: ReportedSpanCategory): string {
+  return CATEGORY_COLOR[category](`[${category}]`);
+}
+
+function loopBusy(node: TraceResult): string | null {
+  const { elu } = node;
+  // a sync span never yields to the loop, so the number would be a trivial 100%
+  if (!elu || node.sync) {
+    return null;
+  }
+  const total = elu.idle + elu.active;
+  if (total < 1) {
+    return null;
+  }
+  return `loop-busy=${fmtPercent(elu.active, total)}`;
+}
+
+function threadLabel(thread: number): string {
+  return thread === 0 ? 'main' : `worker#${thread}`;
+}
+
+/**
+ * Collapse the middle of a breadcrumb so both ends survive: the task it belongs
+ * to and the span itself (plus as many of its nearest ancestors as fit).
+ */
+function fmtPath(path: string[], maxLength: number): string {
+  if (path.length < 3) {
+    return path.join(' › ');
+  }
+  const head = path[0];
+  let out = path.at(-1)!;
+  for (let i = path.length - 2; i >= 1; i--) {
+    const candidate = `${path[i]} › ${out}`;
+    if (head.length + candidate.length + 8 > maxLength) {
+      return `${head} › … › ${out}`;
+    }
+    out = candidate;
+  }
+  return `${head} › ${out}`;
+}
+
+/** Printable width of a string that may carry picocolors escapes */
+function visibleLength(s: string): number {
+  return stripVTControlCharacters(s).length;
+}
+
+function pad(s: string, width: number, alignRight: boolean): string {
+  const fill = ' '.repeat(Math.max(0, width - visibleLength(s)));
+  return alignRight ? fill + s : s + fill;
+}
+
+function table(header: string[], rows: string[][], rightAlignFrom = 1): string[] {
+  const widths = header.map((h, col) => Math.max(visibleLength(h), ...rows.map(r => visibleLength(r[col]))));
+  const fmtRow = (row: string[]) => row
+    .map((cell, col) => pad(cell, widths[col], col >= rightAlignFrom))
+    .join('  ');
+  return [
+    picocolors.bold(fmtRow(header)),
+    picocolors.dim(widths.map(w => '─'.repeat(w)).join('  ')),
+    ...rows.map(fmtRow)
+  ];
+}
+
+// ---------------------------------------------------------------------------
+// Printing
+// ---------------------------------------------------------------------------
+
+export function printTraceResult(traceResult: TraceResult) {
+  printTree(
+    normalizeTraceClock(traceResult),
+    (node, parentThread) => {
+      const parts: string[] = [node.name];
+
+      if (node.category) {
+        parts.push(categoryTag(node.category));
+      }
+
+      if (!isFinished(node)) {
+        parts.push(picocolors.red('(unfinished)'));
+        return parts.join(' ');
+      }
+
+      parts.push(picocolors.bold(fmtMs(node.end - node.start)));
+
+      if (node.children.length > 0) {
+        parts.push(picocolors.dim(`self=${fmtMs(selfTime(node))}`));
+      }
+
+      const busy = loopBusy(node);
+      if (busy) {
+        parts.push(picocolors.dim(busy));
+      }
+
+      if (node.thread !== parentThread) {
+        parts.push(picocolors.dim(`@${threadLabel(node.thread)}`));
+      }
+
+      return parts.join(' ');
+    }
+  );
+}
+
+function printTree(initialTree: TraceResult, printNode: (node: TraceResult, parentThread: number) => string) {
+  function printBranch(tree: TraceResult, branch: string, isGraphHead: boolean, isChildOfLastBranch: boolean, parentThread: number) {
+    const children = tree.children;
+
+    let branchHead = '';
+
+    if (!isGraphHead) {
+      branchHead = children.length > 0 ? '┬ ' : '─ ';
+    }
+
+    console.log(`${branch}${branchHead}${printNode(tree, parentThread)}`);
+
+    let baseBranch = branch;
+
+    if (!isGraphHead) {
+      baseBranch = branch.slice(0, -2) + (isChildOfLastBranch ? '  ' : '│ ');
+    }
+
+    const nextBranch = `${baseBranch}├─`;
+    const lastBranch = `${baseBranch}└─`;
+
+    children.forEach((child, index) => {
+      const last = children.length - 1 === index;
+      printBranch(child, last ? lastBranch : nextBranch, false, last, tree.thread);
+    });
+  }
+
+  printBranch(initialTree, '', true, false, initialTree.thread);
+}
+
+/** Gantt-style overview of the top-level tasks on a shared timeline */
+export function printStats(rawStats: TraceResult[]): void {
+  const stats = rawStats.reduce<TraceResult[]>((acc, trace) => {
+    const normalized = normalizeTraceClock(trace);
+    if (isFinished(normalized)) {
+      acc.push(normalized);
+    }
+    return acc;
+  }, []);
+  if (stats.length === 0) {
+    return;
+  }
+
+  const longestTaskName = Math.max(...stats.map(i => i.name.length));
+  const realStart = Math.min(...stats.map(i => i.start));
+  const realEnd = Math.max(...stats.map(i => i.end));
+
+  const width = 100;
+  const statsStep = Math.max((realEnd - realStart) / width, 1);
+
+  console.log(picocolors.bold('[timeline]'), `${fmtMs(realEnd - realStart)} wall, one column ≈ ${fmtMs(statsStep)}`);
+
+  stats
+    .sort((a, b) => a.start - b.start)
+    .forEach((stat) => {
+      const offset = ((stat.start - realStart) / statsStep) | 0;
+      const length = Math.max(((stat.end - stat.start) / statsStep) | 0, 1);
+      console.log(
+        `[${stat.name}]${' '.repeat(longestTaskName - stat.name.length)}`,
+        ' '.repeat(offset) + '='.repeat(length),
+        picocolors.dim(fmtMs(stat.end - stat.start) + (stat.thread === 0 ? '' : ` @${threadLabel(stat.thread)}`))
+      );
+    });
+}
+
+export interface BuildResourceUsage {
+  /** Main thread event loop utilization delta over the whole build */
+  elu?: SpanEventLoopUtilization,
+  /** `process.cpuUsage()` delta over the whole build, in microseconds. Covers every thread */
+  cpu?: NodeJS.CpuUsage
+}
+
+function printOverview(analysis: TraceAnalysis, usage: BuildResourceUsage | undefined) {
+  const parts = [`wall=${fmtMs(analysis.wall)}`];
+
+  if (usage?.elu) {
+    const { active, idle } = usage.elu;
+    parts.push(`main-loop-busy=${fmtMs(active)} (${fmtPercent(active, active + idle)})`, `main-loop-idle=${fmtMs(idle)}`);
+  }
+  if (usage?.cpu) {
+    const user = usage.cpu.user / 1000;
+    const system = usage.cpu.system / 1000;
+    const total = user + system;
+    parts.push(
+      `process-cpu=${fmtMs(total)} (user ${fmtMs(user)} / sys ${fmtMs(system)})`,
+      analysis.wall > 0 ? `cpu/wall=${(total / analysis.wall).toFixed(2)}x` : ''
+    );
+  }
+  if (analysis.unfinishedSpans > 0) {
+    parts.push(picocolors.red(`unfinished-spans=${analysis.unfinishedSpans}`));
+  }
+
+  console.log(picocolors.bold('[build]'), parts.filter(Boolean).join(' '));
+}
+
+function printCategoryBreakdown(analysis: TraceAnalysis) {
+  const grandTotal = analysis.categories.reduce((acc, c) => acc + c.selfTotal, 0);
+
+  console.log();
+  console.log(picocolors.bold('[time by category]'));
+  console.log(picocolors.dim(
+    '  self = span time not covered by traced children, summed across concurrent spans (so it exceeds wall;\n'
+    + '         an async span on a busy event loop also includes time queued behind other work);\n'
+    + '  coverage = wall-clock during which at least one span of that category was in flight;\n'
+    + '  sync = the part of self that came from synchronous spans, i.e. guaranteed CPU on that thread'
+  ));
+
+  const rows = analysis.categories.reduce<string[][]>((acc, c) => {
+    if (c.spans > 0) {
+      acc.push([
+        CATEGORY_COLOR[c.category](c.category),
+        fmtMs(c.selfTotal),
+        fmtPercent(c.selfTotal, grandTotal),
+        fmtMs(c.coverage),
+        fmtPercent(c.coverage, analysis.wall),
+        String(c.spans),
+        fmtMs(c.selfOnMainThread),
+        fmtMs(c.selfOnWorkers),
+        fmtMs(c.selfSync)
+      ]);
+    }
+    return acc;
+  }, []);
+
+  table(['category', 'self', 'share', 'coverage', 'of wall', 'spans', 'main', 'workers', 'sync'], rows)
+    .forEach(line => console.log('  ' + line));
+}
+
+function printTopSpans(analysis: TraceAnalysis) {
+  console.log();
+  console.log(picocolors.bold('[top spans by self time, per category]'));
+  for (let i = 0, len = analysis.categories.length; i < len; i++) {
+    const stat = analysis.categories[i];
+    if (stat.top.length === 0) {
+      continue;
+    }
+    console.log('  ' + categoryTag(stat.category));
+    for (let j = 0, jlen = stat.top.length; j < jlen; j++) {
+      const span = stat.top[j];
+      const extra: string[] = [];
+      if (span.node.children.length > 0) {
+        extra.push(`wall=${fmtMs(span.duration)}`);
+      }
+      const busy = loopBusy(span.node);
+      if (busy) {
+        extra.push(busy);
+      }
+      if (span.node.thread !== 0) {
+        extra.push(`@${threadLabel(span.node.thread)}`);
+      }
+      console.log(
+        '    ',
+        picocolors.bold(fmtMs(span.self).padStart(9)),
+        fmtPath(span.path, 110),
+        extra.length ? picocolors.dim(extra.join(' ')) : ''
+      );
+    }
+  }
+
+  console.log();
+  console.log(picocolors.bold('[top spans by self time, overall]'));
+  for (let i = 0, len = analysis.topSpans.length; i < len; i++) {
+    const span = analysis.topSpans[i];
+    console.log(
+      '    ',
+      picocolors.bold(fmtMs(span.self).padStart(9)),
+      categoryTag(span.category).padEnd(16),
+      fmtPath(span.path, 110)
+    );
+  }
+}
+
+function printTaskMatrix(analysis: TraceAnalysis) {
+  if (analysis.tasks.length === 0) {
+    return;
+  }
+
+  console.log();
+  console.log(picocolors.bold('[time by task × category]'), picocolors.dim('(self time of the task and all its descendants)'));
+
+  const usedCategories = CATEGORY_ORDER.filter(c => analysis.tasks.some(t => t.byCategory[c] > 0));
+
+  const rows = analysis.tasks
+    .toSorted((a, b) => b.duration - a.duration)
+    .map((task) => {
+      const busy = task.node.elu ? fmtPercent(task.node.elu.active, task.node.elu.active + task.node.elu.idle) : 'n/a';
+      return [
+        task.node.name + (task.node.thread === 0 ? '' : picocolors.dim(` @${threadLabel(task.node.thread)}`)),
+        fmtMs(task.duration),
+        busy,
+        ...usedCategories.map(c => (task.byCategory[c] > 0 ? fmtMs(task.byCategory[c]) : picocolors.dim('·')))
+      ];
+    });
+
+  table(['task', 'wall', 'loop-busy', ...usedCategories], rows)
+    .forEach(line => console.log('  ' + line));
+}
+
+/**
+ * The full post-build report: every task's span tree, then the aggregate views
+ * (where does time go by category, which spans dominate, per-task breakdown and
+ * the shared timeline).
+ */
+export function printBuildReport(traces: TraceResult[], usage?: BuildResourceUsage) {
+  traces.forEach(printTraceResult);
+
+  const analysis = analyzeTraces(traces);
+
+  console.log();
+  printOverview(analysis, usage);
+  printCategoryBreakdown(analysis);
+  printTopSpans(analysis);
+  printTaskMatrix(analysis);
+  console.log();
+  printStats(traces);
+}
diff --git a/Build/trace/types.ts b/Build/trace/types.ts
new file mode 100644
index 00000000..12cedad7
--- /dev/null
+++ b/Build/trace/types.ts
@@ -0,0 +1,68 @@
+import type { EventLoopUtilization } from 'node:perf_hooks';
+
+export const SPAN_STATUS_START = 0;
+export const SPAN_STATUS_END = 1;
+
+/**
+ * What a span spends its *self* time on (its own duration minus whatever its
+ * traced children cover). Unmarked spans are reported as "uncategorized" rather
+ * than inheriting from their parent, so gaps in instrumentation stay visible.
+ */
+export enum SpanCategory {
+  /** CPU-bound work on the current thread (parsing, trie ops, hashing, formatting) */
+  Compute = 'compute',
+  /** Reading from the local filesystem */
+  FsRead = 'fs-read',
+  /** Writing to the local filesystem */
+  FsWrite = 'fs-write',
+  /** Fetching from the network (download, DNS, HTTP HEAD, ...) */
+  Network = 'network',
+  /** Handing work to / waiting on a worker thread. Set automatically by traceWorkerChild */
+  Worker = 'worker',
+  /** Waiting on another in-flight promise that is traced (or untraced) elsewhere */
+  Wait = 'wait'
+}
+
+export const UNCATEGORIZED = 'uncategorized';
+export type ReportedSpanCategory = SpanCategory | typeof UNCATEGORIZED;
+
+/** Delta of `performance.eventLoopUtilization()` across the span, in milliseconds */
+export interface SpanEventLoopUtilization {
+  idle: number,
+  active: number
+}
+
+export interface TraceResult {
+  name: string,
+  category?: SpanCategory,
+  start: number,
+  end: number,
+  /**
+   * Event loop utilization of the thread that ran this span, over the span's
+   * lifetime. `active` is an upper bound of this span's own CPU time: on a shared
+   * event loop it also includes work done by concurrently running spans.
+   */
+  elu?: SpanEventLoopUtilization,
+  /**
+   * Set when the span wrapped a synchronous function: its whole self time is
+   * CPU time on its thread, never queueing behind other work on the event loop.
+   */
+  sync?: true,
+  /** `worker_threads.threadId` of the thread that created the span (0 = main thread) */
+  thread: number,
+  /**
+   * `performance.timeOrigin` of the thread that produced this trace. Only set on
+   * the root returned by a task so traces produced on a worker thread can be
+   * shifted onto the main thread's clock before being compared with others.
+   */
+  timeOrigin?: number,
+  children: TraceResult[]
+}
+
+/** Pure data object — safe to transfer across Worker Thread boundaries. */
+export interface RawSpan {
+  traceResult: TraceResult,
+  status: typeof SPAN_STATUS_START | typeof SPAN_STATUS_END,
+  /** `performance.eventLoopUtilization()` sampled when the span started */
+  eluStart?: EventLoopUtilization
+}