Google changes cumulative layout change metric

Google has made changes to how it calculates CLS, cumulative layout change, metric in core web vitals. Specifically, maximum session window with 1 second interval, capped at 5 seconds. This was done to make it “fairer for long pages and single-page apps,” Google’s Malte Ubl said.

Google said it decided to move forward with smaller, capped maximum windows. This can impact your CLS scores in key web vitals, Google explained:

Since this update caps a page’s CLS, no page will score worse as a result of this change. And based on our analysis, 55% of origins will see no CLS change at the 75th percentile. This is because their pages currently have no layout changes or the changes they 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 around 3% will see their scores go from a “needs improvement” or “poor” rating to a “good” rating. These pages tend to use infinite scrollers or have many slow UI updates, as described in our previous article.

Here’s a bit of what it looks like when it does its exam session windows:

You can learn many more technical details this way.

Update: Google job that since April 13, 2021, it has updated these statistics in the Search Console report. Google wrote “CLS metrics have been updated to reflect a more accurate representation of on-page layout changes. You may see changes in your page’s CLS statuses (mostly positive) reflecting this change.”

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