HTML document size on React sites

Sites with over 75 KB of HTML initial HTML document size have a good LCP most often: 84%, against 79% of all React sites. Sites with 30 to 75 KB of HTML HTML size have it least often, at 66%.

Sites with 30 to 75 KB of HTML initial HTML document size have a good INP most often: 99%, against 99% of all React sites. Sites with under 14 KB of HTML HTML size have it least often, at 97%.

Over 75 KB of HTML is the most used: 67.7% of React sites.

Across the four LCP sub-parts in Chrome's field data, the largest gap between over 75 KB of HTML and 30 to 75 KB of HTML sits in time to first byte (TTFB): 507 ms median for over 75 KB of HTML against 1053 ms for 30 to 75 KB of HTML. Overall median field LCP is 1.4s for over 75 KB of HTML (TTFB 507 ms, load delay 480 ms, load duration 184 ms, render delay 157 ms) versus 2.0s for 30 to 75 KB of HTML.

Across the four LCP sub-parts in Chrome's field data, the largest gap between over 75 KB of HTML and 30 to 75 KB of HTML sits in time to first byte (TTFB): 507 ms median for over 75 KB of HTML against 1053 ms for 30 to 75 KB of HTML. Overall median field LCP is 1.4s for over 75 KB of HTML (TTFB 507 ms, load delay 480 ms, load duration 184 ms, render delay 157 ms) versus 2.0s for 30 to 75 KB of HTML.

Wix sets the initial HTML document size to over 75 KB of HTML on 98% of its sites, and 92% of Wix sites have a good LCP, against 79% of all sites. WordPress sets the initial HTML document size to 30 to 75 KB of HTML on 54% of its sites, and 66% of WordPress sites have a good LCP.

Wix sets the initial HTML document size to over 75 KB of HTML on 98% of its sites, and 100% of Wix sites have a good INP, against 99% of all sites. WordPress sets the initial HTML document size to 30 to 75 KB of HTML on 54% of its sites, and 100% of WordPress sites have a good INP.

PlatformInitial HTML document size on its sitesGood LCP

All React sites 9,570 sitesOver 75 KB 68%30 KB to 75 KB 17%79%

Wix 5,824 sitesOver 75 KB 98%92%

WordPress 2,342 sites30 KB to 75 KB 54%14 KB to 30 KB 20%Over 75 KB 20%66%

Over 75 KB 30 KB to 75 KB 14 KB to 30 KB Under 14 KB (First TCP window)

Does the platform explain the initial HTML document size difference?

On WordPress, 62% of sites with over 75 KB of HTML initial HTML document size have a good LCP, against 62% of WordPress sites with 30 to 75 KB of HTML. Within WordPress, sites with over 75 KB of HTML and 30 to 75 KB of HTML have a good LCP about equally often.

On WordPress, 100% of sites with 30 to 75 KB of HTML initial HTML document size have a good INP, against 99% of WordPress sites with over 75 KB of HTML. Within WordPress, sites with 30 to 75 KB of HTML and over 75 KB of HTML have a good INP about equally often.

Why the 14 KB initial TCP congestion window sets the pace for LCP discovery

A new TCP connection transfers roughly 14 KB (10 segments) in its first round-trip before waiting for a client ACK. Across 548 sites (5.7%) whose HTML fits under 14 KB, 81% achieve good LCP (1.6s median, 569 ms TTFB). Sites between 14 and 30 KB (9.7%) record 83% good LCP (1.5s median), while pages bloating to 30 to 75 KB (16.9%) drop to 66% good LCP (1.9s median) and pages over 75 KB (67.7%) sit at 84% good LCP (1.4s median).