Checks
Cumulative Layout Shift
Grades Cumulative Layout Shift against Google's Core Web Vitals thresholds, passing at 0.1 or below and failing beyond 0.25.
What this check looks for
For pages with measured performance data, this check grades CLS — a score of how much visible content unexpectedly shifts position while the page loads — against Google's thresholds: at or under 0.1 passes, between 0.1 and 0.25 warns, and beyond 0.25 fails. Pages without a measured CLS value report "na".
Why it matters
Layout shift is what happens when an image, ad, or embed loads in after the surrounding text has already rendered, pushing everything below it down the page — often at the exact moment a visitor was about to tap a button, causing a frustrating mis-click on something else entirely. It's one of Google's three Core Web Vitals and a direct ranking factor, so a page that visibly jitters while loading is penalized independently of how good its content actually is. A visually unstable page also just reads as unpolished and unfinished, which subtly undermines trust in the content itself even when the instability is purely cosmetic.
How to fix it
Reserve space for anything that loads asynchronously, before it arrives:
<!-- No reserved space: image pops in and pushes content down -->
<img src="/hero.webp" alt="Acme wireless headphones"><!-- Fixed: width/height reserve the aspect ratio before the image loads -->
<img src="/hero.webp" alt="Acme wireless headphones" width="1600" height="900">Apply the same principle to ads, embeds, and any dynamically-inserted banners — reserve a fixed-height container for them rather than letting content reflow once they load.
See how your site scores on this check