[SOLVED] Page speed LAB data is not reflecting on FIELD data

Page speed LAB data is not reflecting on FIELD data

I am facing some issue with the LAB data and FIELD of google page speed report. Here is my site URL- https://www.accuride.com/en-us. From April to May I am having a LAB data score around 80-90 but the FIELD data is showing very poor - FCP 2.6 and LCP 6. From June I am having a poor LAB data score as well 70-80 but the FIELD data starts improving - FCP 2.2 and LCP 4.7. Now I am having a lab score around 65-75 but FIELD data is improved.

I am currently confused about the results which data I will follow to improve the site. Can any one suggest on this?

Solution

Field data is what your users experience, and what feeds into the Core Web Vitals initiative.

Lab data is what that particular run of Lighthouse experienced. It does a cold load of your website, under specific conditions that may or may not be representative of your site.

There are many reasons why they might be different:

Your users may typically be faster than lab-based Lighthouse run was completed under.
Your users may typically be slower than lab-based Lighthouse run was completed under.
Your users may be doing less cold loads (e.g. have cached assets either from previous visits or from browsing around your site).
Your users may be based far away from your servers, or follow multiple redirects to get there, compared to Lighthouse which might be going there directly from servers with faster connections to your servers.
Your users may scroll or otherwise interact with your site while Lighthouse does not.

In short concentrate on the field values and use the lab data as diagnostic information that could help you improve that.

However, in this case both are telling you that you have a problem, when I run that URL through PageSpeed Insights. And particularly on mobile. Yes lab might be saying you have a bigger problem than field, but they are both saying you have a problem.

Be aware that the field data is actually up to 4 pieces of field data (mobile for the URL entered and the origin, and same for desktop), so understand how to read it. Good details on a post I help maintain here: https://web.dev/optimize-lcp/#using-pagespeed-insights-crux-lcp-data

The Lighthouse score is intended as a high-level summary of what the lab report is. Personally I'd concentrate on that less and look at the numbers. There's good details on what to do with these numbers in the Google docs (that, full disclosure, I help to maintain)