Why 60fps feels like 24fps

The eye forgives slow. It does not forgive uneven.

Film has run at 24 frames per second for a century and nobody calls it janky, because every frame arrives exactly when expected — 41.7ms apart, no exceptions. A web animation at a steady 30fps reads as smooth but slightly dreamy. The same animation averaging 30fps with frames arriving at 8ms, 40ms, 12ms, 55ms reads as broken — because the brain does not perceive frame rate, it perceives *gaps*.

Jank is not 'low fps'. Jank is a frame that arrives late, especially after a run of on-time frames. The eye has a rhythm detector, and a single late frame is a skipped heartbeat.

Where the gaps come from

• Main-thread work: layout and paint are the usual suspects — animating width or top forces layout every frame, and layout takes whatever time the DOM gives it.
• Compositor interruptions: a layer is promoted mid-animation, or a new layer appears (a dropdown opening under the animation) and the GPU re-composites.
• Garbage collection: a rAF loop that allocates objects every frame (strings, arrays) makes the GC pause the main thread unpredictably.
• Background tabs waking up, extension work, or a font swap that triggers a reflow mid-sequence — the classic invisible jank.
• Scroll-handler pileup: a scroll listener that writes layout (reads offsetTop, then writes style) forces synchronous reflow on every scroll event.

16.7ms  total budget at 60fps (one frame)
 8ms    style + layout          <- keep tiny
 3ms    paint
 4ms    compositor
 1ms    JavaScript
-----
Keep JS under 4-5ms per frame and the
rest of the budget survives real devices.

Why the fix is usually fewer moving parts

The cheapest way to a steady 60 is to animate only transform and opacity (compositor-only, no layout, no paint) and to promote long-running layers explicitly with will-change. But the deeper fix is humility: a page where four things animate at once will fight for the budget forever. Cut the animation to the one that tells the story, and steady frames come back — because the fastest frame is the one you did not ask for.