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Lab data, as well as online analysis tools show my site to have 0 cumulative layout shift (CLS) issues in mobile and desktop versions. However, Google field data is showing 0.30 CLS issues, (Critical, no passing level!).

Is this to be expected and typical? I have no other ideas on how to fix this. I wonder how people can still experience CLS.... perhaps they are on older devices that don't respond to modern code, therefore having CLS issues?

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  • Have you used Google Chrome's Lighthouse developer tools to test your site yourself with a variety of screen widths? The lab data from Pagespeed Insights only uses two widths: one for desktop and one for mobile. It is possible that there is some layout shift at another browser width. – Stephen Ostermiller Jan 27 at 11:12
  • @StephenOstermiller, are you saying that Lighthouse tests CLS across all resolutions in a single step or that you can simply change the viewport and retest? – Trebor Feb 19 at 15:31
  • Using the Chrome developer tools you can change the viewport and rerun the lighthouse tests yourself. – Stephen Ostermiller Feb 19 at 16:02
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Is this to be expected and typical? In short, yes.

Google's extensive documentation on Core Web Vitals, and specifically on cumulative layout shift, are explicit on this point:

Caution: Lab tools typically load pages in a synthetic environment and are thus only able to measure layout shifts that occur during page load. As a result, CLS values reported by lab tools for a given page may be less than what real users experience in the field.

Source: https://web.dev/cls/

[…] while testing in the lab is a reasonable proxy for performance, it isn't necessarily reflective of how all users experience your site in the wild.

Source: https://web.dev/user-centric-performance-metrics/#in-the-field

Your "lab" measurements are useful up to a point, but the real measurement is the aggregated, real-world experiential data coming from CrUX (Chrome User Experience Report).

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