What is CLS (Cumulative Layout Shift)?

CLS (Cumulative Layout Shift) is a metric that measures how much and how unpredictably page elements shift during loading.

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#What it looks like:

• you are reading text, and suddenly everything jumps down because an ad loaded;
• you want to tap a button, but at the last moment it moved, and you tapped the wrong thing.

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#Why it is bad:

Shifts interfere with interaction, annoy users, and lead to mistaken actions. Especially on mobile.

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#Good scores:

• up to 0.1 is considered normal;
• 0.1-0.25 needs attention;
• more than 0.25 is a poor user experience.

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CLS does not depend on loading time. It is a matter of layout stability, not speed. The fewer the jumps, the better the perception of the site and the higher user loyalty.