Load the product page on a phone and watch for anything that moves after it appears. Google counts CLS of 0.1 or less as good. Banners and late images cause most of it.
web.dev sets good Cumulative Layout Shift at 0.1 or less and poor above 0.25, at the 75th percentile of page loads.
Announcement bars that slide in, review stars that load late, images without set sizes, and fonts that swap. A theme that stays still while it loads should rank above one with a nicer demo.
Source: web.dev, Cumulative Layout Shift (CLS). Reviewed Sep 23, 2026.