The Frame Budget (60fps)
To hit 60 frames per second, the browser has roughly 16 ms per frame for JS, style, layout, paint, and composite combined. Overrun it and frames drop.
A single 50 ms task blows the 16 ms budget and drops about three frames.
work per frame16 ms budget
8 ms task โ fits the frame
Tap the example to replay it ยท press and hold to slow it down