Glossary
CLS (Cumulative Layout Shift)
How much the page jumps around while it loads.
What it is
Cumulative Layout Shift scores how much visible content moves unexpectedly during loading. A banner that pushes the content down, an image that arrives without reserved space, a font swap that reflows a paragraph — each contributes.
Why it costs you money
Layout shift causes mis-taps, and on a store a mis-tap can mean adding the wrong variant or leaving the page. It is also the cheapest Core Web Vital to fix, because it is usually caused by missing image dimensions rather than by anything architectural.
How to check it yourself
Check the field data, then look for images and embeds without width and height attributes, and for anything injected above existing content after load — cookie bars and promotional strips are frequent culprits.