WPT chooses different LCP than Pagespeed Insights

[RogerM]

Perhaps this will help…

Is LCP calculated differently on WPT than on Lighthouse?

Recently, I noticed that WPT can give you some useful information about LCP. It can identify the type of LCP, in my case it was an image LCP, and it also identified that image’s URL. I optimized this LCP image and that improved LCP score in WBT. I then ran separate tests on Lighthouse, but on that tool the score decreased after having optimized the LCP image. Then I ran some WPT tests, with the Lighthouse audit option turned on, and I did notice that the LCP scores that show in the UI did no…