CLS Confusion

[Ray Amsterdam]

Hi .... on this site https://barkco.com

Every page examined gets a score of between 90 and 100 - and the CLS “live” score nearly always comes back with a time 0 or very close to 0 (always green).

Even on the only page that has a low score (the contact page) that has lots of scripts on it (ironically mainly from Google themselves) the CLS score is still 0 (green). 

Can someone therefore please explain why the CLS score on the Core Web Vitals Assessment is getting a score of 0.34 (a “fail”) but the CLS score on the live test for the pages is not failing - it is green with a time at 0!

Many thanks!

Ray

[Michal Mocny]

CLS measures all layout shifts over the lifetime of the page, but many automated tests like lighthouse or Page Speed Insights only test loading performance.  Is it possible that this site has 0 layout shifts during load, but either has shifts coming very late, or very slowly after user interactions?

-Michal

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[Philip Walton]

Hi Ray,

In addition to what Michal said, I believe this post will help answer your question:

https://web.dev/lab-and-field-data-differences/#cls

Feel free to reply here if you have any follow up questions.

To view this discussion on the web visit https://groups.google.com/d/msgid/web-vitals-feedback/CAEeF2TeHzmsOM1gE7Y2Zoti21cjGWQm2ctM%3D8GnDKTNb57YEKg%40mail.gmail.com.

[Ray Amsterdam]

Hi All,

I think I managed to work out the issues that were causing the CLS issue - thanks to you.  But maybe not?

Do you know if the "Web Vitals" extension for Chrome that was kindly suggested here and which I used, is the only investigative tool?  Do I now need to wait a month or so for Google to reissue its verdict?

[Barry Pollard]

Measuring the web vitals yourself using a RUM solution is the best way to have a detailed view and more control than CrUX will be able to give you.

However, for a quick view look at the field data in PageSpeed Insights and click on the expanded view. You will then be able to see what percentage of page views, as well as the p75 score. While the score will take a while to change until a sufficient number of the newer, better, page views replace the older, worse one. However, the percentage of page views in each category should see changes every day. This can be a good gauge as to whether you are trending in the right direction (i.e. getting an increasing percentage of "good" page views).

I see at the moment you have 51% Good CLS page view on mobile, 2% Needs Improvement, and 47% Poor. If you run the same test tomorrow, then hopefully you will see that shift to 54%/2%/44% for example. If so, then once the "Good" views reach 75% then you should see the score change and the status changing to pass.

Thanks,

Barry

[Mathieu Beaujard]

Hi, 

Just read that conversation and was wondering what were your recommendations in terms of RUM solutions?

Thanks

[Barry Pollard]

It would be unfair of me to recommend one solution over the others. There are many great commercial solutions to suit your needs and budget. It is even possible to collect your own RUM data through browser APIs, or using the Google web-vitals.js library (https://github.com/GoogleChrome/web-vitals).

Thanks,

Barry

[Mathieu Beaujard]

Ok thanks anyway,

Mat

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