CrUX API updates to LCP, CLS, and FCP

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Rick Viscomi

Jun 2, 2021, 3:36:48 PM6/2/21
to Chrome UX Report (Announcements)
Hi everyone,

The CrUX API has been updated to include changes to several metrics. Please read this blog post for a timeline of changes and more information about how this fits in with the overall Web Vitals tooling suite. Here's a summary of what's new for each metric:

Evolving the CLS metric describes the implementation changes to CLS, most notably a maximum session window of 5 seconds with a 1 second gap. This change should strictly improve your CLS values if there's any change at all.

The previous CLS implementation has been forked and made temporarily available in the API as `experimental_uncapped_cumulative_layout_shift` so you can continue to track changes using the old version if needed.

The LCP change log describes the implementation changes to LCP, most notably to consider elements that have been removed from the DOM as eligible. You may see gradual improvements to LCP as the new metric takes effect, depending on how much your site is affected by removed LCP elements.

The FCP "good" threshold has been changed from 1.0 to 1.8 seconds. This is a more achievable threshold so you may see significant improvements to your site's FCP.

We'll post another update next week during the regularly scheduled monthly BigQuery release with more information about how these changes affect the CrUX dataset on BigQuery.

For general feedback/questions about these Web Vitals metrics, reach out to the web-vitals-feedback group. For CrUX-specific feedback, reach out to the CrUX Discuss group.


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