While general 6.2 performance Web Vitals are already available in this spreadsheet, the test here was conducted specifically for a block theme (TT3) single post with a core/image block in it.

In WordPress 6.1 this scenario had a bug that it would lazy-load the image even though it is the first content image. This bug has been fixed via #56930 for WordPress 6.2.

Web Vitals were gathered similar to the ones in the spreadsheet linked above, by issuing 20 requests for each test scenario, using this CLI script.

Since this fix is specifically client-side performance, looking just at LCP may be deceiving, since that metric also includes TTFB (i.e. the server response time), which is not directly affected by this change.

TTFB is seeing a major improvement in WordPress 6.2, but here we need to ignore that to get a better idea on the impact of this particular fix alone.

A good approach to at least get a better idea on the client-side performance is to use LCP - TTFB. This data has been calculated below.

Comparing these numbers with a quick LCP - TTFB calculation from the general WordPress 6.2 performance Web Vitals data, which was tested without an image in the content, the improvement here is notably greater.

For the overall metrics, the median LCP - TTFB is ~106ms for WP 6.1.1 and ~102ms for WP 6.2 Beta 3, so that's only a ~4% improvement, far less than the ~13% we are seeing here.