Difference between Core Web Vitals & LHS metrics

[Meet Kumar Jain]

Hello TEAM, 

For any page speed stats let say - https://pagespeed.web.dev/analysis/https-www-gigamon-com/ycyrcbu4y5?form_factor=mobile

We have Core Web Vitals section and we have LHS metrics down below, where it does again talk about LCP, CLS, FCP etc but not FID. 

Which stats we need to concentrate on ? 28 days data by CRUX or the real time data under LHS metrics ? 

Regards,

Meet

[Barry Pollard]

Always concentrate on field data (the top section) for how your site is actually experienced by your actual users.

Use the Lab data (Lighthouse data at the bottom) to drill down into that for more diagnostic details. But always check it matches up to the field data to see how representative the Lighthouse test is of your real user. There is likely less to gain from optimising LCP when Lighthouse tells you that is an issue, if your real users are not experiencing any LCP problems. Yes you can always improve, but you should look at other potential problem areas first.

To pick two quotes from this article, which explains all the Google tools that provide Core Web Vitals data, and what they are good for:

Lab data describes how hypothetical users may experience your website. Field data describes how real users actually experienced your website. Field data is also known as Real User Monitoring (RUM), and is typically collected by monitoring real user experiences using JavaScript on the pages they load, and reporting various metrics to an analytics solution.

and

Always concentrate on field Core Web Vitals over Lighthouse metrics and scores. In particular, the Performance Score of Lighthouse is a broad measure of that lab test and often does not correlate with field Core Web Vitals.

Lighthouse is an amazingly useful tool to look at ways you can improve your site and repeatedly test changes quickly. But is a single, cold, page load, until very specific conditions (often slower than real users to emphasize performance problems), rather than a good measuring tool of how the site is actually used and experienced.

As Lighthouse does not by default do any interactions a users would do (open menus, click buttons, click links) it cannot measure FID nor INP. Total Blocking Time (TBT) is the best it can surface so show when the page is running a lot of blocking code and therefore likely to experience FID and INP issues.

Barry

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