Frontend & browser·Performance and Core Web Vitals
the scroll is smooth and then jumps, because a frame took too long and got skipped.
Jank
Also calledstutter, dropped frames
Visible unevenness caused by frames missing their deadline, whether from main-thread work, expensive layout or too much painting. Users notice irregularity more than they notice a consistently lower frame rate, so one dropped frame in a smooth scroll is worse than a uniformly slower animation. The profiler's frame view is the fastest way to find which frame ran long and what it was doing.