Largest Contentful Paint (LCP) / Pagespeed Insights

[Chris Ratchford]

I've noticed in Google Pagespeed Insights, when optimizing a site (hosted by WPEngine) using WP Rocket (Pro version- paid) for pagespeed, the Largest Contentful Paint (LCP) number never fluctuates, regardless of any caching (or minifying css/js/html). But there's a second instance of LCP number further down the report (see screenshot), that does fluctuate.

Looking at the report, there's 4.2 LCP, then another one at 3.4 s LCP. Is this tool broken? Any explanation why there'd be conflicting data?

 

[Patrick Stox]

The top number is from real-world user data (Chrome User Experience Report CrUX data). I can't tell from the screenshot if it's from the page level data or the origin data. There are tabs that should show which it is. This is how users experience the page or site on average and takes into account caching, location, their connection speed, their devices, etc. Notice the details in gray at the bottom of the section?

The bottom is lab test data done with set conditions. This test is done by a system called Lighthouse. It will also tell you the test conditions in the gray section at the bottom not shown in the screenshot. This is a specific location, device, network conditions, etc. Even this number itself can vary a bit and you may need to do 3-5 runs and average the number. A browser can actually make some different decisions when loading the page and conditions and resources can vary slightly. It should be fairly similar each time, but as I said it can be a bit different.

 

[Ljubica]

@Chris Ratchford The data you see first in the report is Field Data, followed by Lab Data.

Field Data capture real-world user experiences. This is the only set of data visible in GSC. It becomes visible only when a URL has had a substantial number of visits over the last 28 days. Any page speed issues are identified and recorded in this data.

Lab Data is a useful tool for diagnosing performance issues, especially if you don't have Field Data (real-user experience). This data, as it is not sourced from actual users, serves as a predictive measure of a website's performance during a user's visit. Google does not consider Lab Data as a factor when determining search rankings.

So always focus on Field Data, if available. In cases where they are not available, remember that Google does not consider page speed (as represented by Lab Data) as a ranking factor