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Honest web frame budgets: one sampler per run, only frames that move, long-animation-frame entries, gzip served

Rule

If frame numbers of a web animation look too good or too bad, run one sampler per run, count only the frames where the animated value changed, and serve the build compressed for load numbers.

Snippet
// before: every requestAnimationFrame interval counted, a sampler per move left running
requestAnimationFrame(function tick(t) { gaps.push(t - last); last = t; requestAnimationFrame(tick); });
// after: one sampler; a frame counts only when the rail (or sheet, or copy) moved since the last
let last = 0, lastP = rail.p();
const tick = (t) => {
  const p = rail.p();
  if (p !== lastP && last) gaps.push(t - last);
  last = t; lastP = p;
  if (running) requestAnimationFrame(tick);
};
// plus PerformanceObserver({ type: 'long-animation-frame' }) overlapping those frames
Check

run the same move twice in a row: the medians agree, and none is near 0 ms (a second sampler halves every interval).

Applies to

other: Chrome's long-animation-frame entries, other: any requestAnimationFrame sampler

I assumed

a requestAnimationFrame sampler per move, and the review server as it was, would give the numbers.

What happened

a second sampler loop left running from the move before halved every interval, giving medians near 0 ms; idle frames between moves diluted the percentiles; and the review server sent the JavaScript uncompressed (2 MB, where a site sends 472 KB), so the first view on fast 4G read 3.1–3.5 s.

The fix

one sampler per run; only frames where the rail's position, the sheet's rise or the bag copy's time changed; long frames from long-animation-frame entries that overlap them; React renders counted through a test hook; idle checked as animation frames and DOM changes over 4 s after every timer; and a small Node server that gzips, keyed by the file's time.

Evidence

the first view with gzip went from 3.1–3.5 s to 1.9–2.8 s on fast 4G; 0 animation frames and 0 DOM changes at rest, in every run (commit 70a7d58).

Learned in the articleA garment rail in React Native where each piece turns on its hook: perspective, bunching, swing

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