Wrong CLS data comes from UX report

[James]

Hello everyone,

I found Chrome UX Report keeps showing wrong data even though I fixed the CLS issue and checked in PageSpeed Insights

https://pagespeed.web.dev/report?url=https%3A%2F%2Flatiste.com%2F

Do you know why and how to fix it?

Thank you.

[Barry Pollard]

PageSpeed Insights lab test does just a load of the page and doesn't scroll or interact with the page in any other way.

The CrUX data (shown at the top if the page) is collected over the previous 28 days from real users - who do scroll, and interact with the page.

For example, if you install the Web Vitals Extension, open dev tools, switch the browser to mobile emulation mode, and view the console, then scroll to the bottom, and then reload the page you see a sizeable CLS:

Now I'm not saying that's necessarily what your site visitors are doing, but that's just the first example I saw of getting CLS - and one that a Lighthouse load at just the top of the page doesn't see.

We recommend measuring Core Web Vitals in the field to allow you to identify issues like this that lab-based tools can easily miss. See this article for more details: https://web.dev/debug-performance-in-the-field/

Thanks,

Barry

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[James]

So CrUX calculates every layout in the same page not just top layout where users will see when they entered the website. It's hard to troubleshoot then.

[Barry Pollard]

Yes CrUX measures the User Experience of the page not just the loading experience. Layout shifts after the page loads are still disconcerting to the user and a sign the page is not performing optimally.

Note that layout shifts as a result of a user interaction are expected and so ignored (as long as they occur within 500ms of that user interaction).