CLS (Cumulative Layout Shift)

CLS is not the size of the worst shift; it is the sum of shift scores across the page's lifetime, where each score multiplies how much of the viewport moved by how far it moved.

That has a practical consequence: many small shifts can score worse than one large one, and shifts that happen after load — a lazy-loaded image arriving, an ad slot filling, a cookie banner appearing — count just as much as those during the initial render. Anything that moves content after the visitor can see it is a candidate.

A shift within 500 milliseconds of a user interaction is excluded, on the reasoning that a person who clicked something expects the page to change. This is why an accordion expanding or a menu opening does not hurt the score.

It also means the fix for some shifts is to tie them to an interaction rather than to eliminate them. Content that must appear late is far less damaging if it appears because someone asked for it.