GUI Performance Profiling via CDP + Playwright

Problem

You have a Three.js / React scene that stutters under interaction. You added [perf] console.warn instrumentation in your render loop. The logs show _animate 165ms but they don't tell you which function inside the frame ate the time. You then spend hours patching plausible suspects (bloom, scene-graph traversal, lazy mesh allocation) and the stutter only partially improves.

The shortcut you missed: Chrome DevTools Protocol gives a real CPU profile in 30 seconds, exposing the actual hot leaf functions.

This is what happened during DAI-22 (atlas stutter hunt). After 4-5 patch iterations on plausible suspects, a CDP profile in one Playwright run revealed animateTo taking 14.4 seconds of self-time in an 8-second window — a function I hadn't even suspected, called from a wholesale for (i=0; i<27149; i++) animateTo(i, ...) selection-clear loop.

Decision rule

If you've spent more than 30 minutes "guessing-and-patching" a perf problem and the next iteration is also a guess — stop and run a CDP profile first. Self-time hot list almost always points at the real culprit; intuitions don't.

Script template (synthetic — driven by the script)

packages/gui/scripts/profile-stutter.mjs:

import { chromium } from 'playwright';
import { writeFileSync } from 'fs';

const argv = process.argv.slice(2);
const secondsArgIdx = argv.indexOf('--seconds');
const seconds = secondsArgIdx >= 0 ? Number(argv[secondsArgIdx + 1]) : 10;
const positional = argv.filter((a, i) => !a.startsWith('--') && argv[i - 1] !== '--seconds');
const url = positional[0] || 'http://localhost:3000/';

const browser = await chromium.launch({ headless: false });
const ctx = await browser.newContext({ viewport: { width: 1600, height: 1000 } });
const page = await ctx.newPage();

page.on('console', (msg) => {
  if (msg.text().startsWith('[perf]')) console.log('  >>', msg.text());
});

await page.goto(url, { waitUntil: 'networkidle', timeout: 60_000 });
// IMPORTANT: wait long enough for the scene to fully load. Headless
// `networkidle` fires before WebGL upload finishes — overshoot deliberately.
await page.waitForTimeout(3_000);

const cdp = await ctx.newCDPSession(page);
await cdp.send('Profiler.enable');
await cdp.send('Profiler.setSamplingInterval', { interval: 100 }); // 100µs
await cdp.send('Profiler.start');

// Drive the interaction. Tune to reproduce the stutter you're hunting.
const canvas = await page.locator('canvas').first().boundingBox();
const cx = canvas.x + canvas.width / 2;
const cy = canvas.y + canvas.height / 2;
const tStart = Date.now();
while ((Date.now() - tStart) / 1000 < seconds) {
  const dx = (Math.random() - 0.5) * 400;
  const dy = (Math.random() - 0.5) * 400;
  await page.mouse.move(cx, cy);
  await page.mouse.down({ button: 'left' });
  for (let i = 1; i <= 10; i++) {
    await page.mouse.move(cx + dx * i / 10, cy + dy * i / 10);
    await page.waitForTimeout(20);
  }
  await page.mouse.up({ button: 'left' });
  await page.mouse.wheel(0, (Math.random() - 0.5) * 400);
  await page.waitForTimeout(100);
}

const { profile } = await cdp.send('Profiler.stop');
writeFileSync('/tmp/stutter.cpuprofile', JSON.stringify(profile));

// Build self-time hot list directly — Chrome DevTools is nice but
// terminal output is faster for triage.
const idToNode = new Map(profile.nodes.map((n) => [n.id, n]));
const selfHits = new Map();
for (const id of profile.samples ?? []) selfHits.set(id, (selfHits.get(id) ?? 0) + 1);
const totalUs = profile.endTime - profile.startTime;
const interval = totalUs / (profile.samples?.length || 1);
const hot = [...selfHits.entries()]
  .map(([id, hits]) => {
    const n = idToNode.get(id);
    return { selfMs: (hits * interval) / 1000, fn: n?.callFrame?.functionName || '(anon)',
             url: n?.callFrame?.url || '', line: n?.callFrame?.lineNumber ?? -1 };
  })
  .sort((a, b) => b.selfMs - a.selfMs).slice(0, 20);
for (const h of hot) {
  console.log(`  ${h.selfMs.toFixed(1).padStart(9)}ms  ${h.fn}  ${h.url.split('/').pop()}:${h.line}`);
}
await browser.close();

Run with node packages/gui/scripts/profile-stutter.mjs --seconds 10 http://localhost:51833/ui/default.

Interactive variant (user drives the action)

When the stutter only happens during a specific interaction the user knows ("when I rotate AND click"), let them drive:

1. Script opens browser, attaches profiler, then polls for a sentinel file: while (!existsSync('/tmp/stutter-stop')) await new Promise(r => setTimeout(r, 500));
2. Tell the user: "потыкай в браузере. Когда наберёшь стоп — touch /tmp/stutter-stop в шелле".
3. On signal: stop profiler, write report, unlink sentinel, close browser.

The user gets to reproduce the exact gesture; you get the profile of that gesture.

Decoding minified function names

Production bundles minify — top hot list shows animateTo HexAtlas-BPUSy0eX.js:4045, useless without source.

Two fixes:

1. Build with sourcemaps for the profiling session. In vite.config.ts set build.sourcemap: true, rebuild, profile. CDP profile carries the original names.
2. Profile against a dev build (vite dev on a separate port). Function names are unmangled.

If neither is available: grep the bundle for the minified token + surrounding context (grep -o ".\{60\}animateTo.\{60\}" dist/assets/*.js) to identify the original.

Reading the profile

• (idle) at #1 = browser is bored. If your script's interaction time was N seconds and (idle) self-time ≈ N seconds, your scene is genuinely fast and the stutter is elsewhere (input handling, layout reflow, scroll).
• (garbage collector) >> 100ms = allocation pressure. Look for new in hot paths.
• A single function holding > 30% self-time = your real bottleneck. Open it and look at how often it's called, not how slow each call is.
• Two entries for the same function (animateTo:4045 × 2) = V8 deopt — the function has two ICs. Same logical function, both contribute.

Common traps the profile exposes

Symptom in hot list	Likely cause
Animation API (animateTo, tween, lerp) at top	Bulk schedule on selection / lens change. Look for for (i=0; i<allNodes; i++)
needsUpdate setter / Float32BufferAttribute	Per-frame buffer reupload of a large array even when 1 element changed
bindVertexArray × thousands	Many small draw calls — merge into BufferGeometry
setMatrixAt / updateMatrixWorld	Scene graph traversal cost — too many Object3Ds even if visible=false
Profiler/(program) >> work	Sampling overhead — normal at 100µs interval

Why this matters

console.warn('[perf]') instrumentation gives per-callback wall-time. CDP gives per-leaf-function self-time. Different granularity, different debug strategy. When a callback is 105ms, instrumentation tells you "callback #0 is slow" — CDP tells you animateTo inside callback #0 is being called 50,000 times.

For a stutter that resists patches: switch granularities. The fix follows in one iteration.