Google has made changes to how it calculates the CLS, cumulative layout shift, metric within core web vitals. Specifically maximum session window with 1 second gap, capped at 5 seconds. This was done to make it “more fair for long running pages and single page apps,” Malte Ubl from Google said.
Google said it decided to move forward with the smaller, capped, maximum windows. This may impact your CLS scores in the core web vitals, Google explained:
Since this update caps the CLS of a page, no page will have a worse score as a result of this change. And based on our analysis, 55% of origins will not see a change in CLS at all at the 75th percentile. This is because their pages either do not currently have any layout shifts or the shifts they do have are already confined to a single session window. The rest of the origins will see improved scores at the 75th percentile with this change. Most will only see a slight improvement, but about 3% will see their scores improve from having a “needs improvement” or “poor” rating to having a “good” rating. These pages tend to use infinite scrollers or have many slow UI updates, as described in our earlier post.
Here is kind of what it looks like when it does its review session windows:
You can learn a lot more technical details over here.
Forum discussion at Twitter.