Google’s new ranking algorithm that launches next year, the Page Experience Update, has metric named CLS, Cumulative Layout Shift. It checks to see if things on the page jump around as it loads. Philip Walton, a Senior Developer Programs Engineer at Google, said that this is measured throughout the entire lifecycle of the page.

He said this during the web.dev Live event at the 1 hour and 57 minute mark. “CLS measures layout shifts that happen during the entire lifecycle of the page,” he said. This is “from when you load the page, until when you unload the page. Even if you leave the page open for days or weeks, it does measure that entire time,” he added.

Here is the video embed at the start time:

Here is the transcript:

So you can’t just run the developer tools on a page for a second or so and expect the score to match what you see in the core web vitals Search Console report.

Hat tip to Glenn:

Also, FID, First Input Delay, is not in Chrome Dev Tools because it requires user interaction, but Total Blocking Time (TBT) is in Dev Tools and it’s a proxy for First Input Delay.

Forum discussion at Twitter.