WordPress HTML document size and the Core Web Vitals

Sites with under 14 KB of HTML initial HTML document size have a good LCP most often: 82%, against 70% of all WordPress sites. Sites with over 75 KB of HTML HTML size have it least often, at 59%.

Sites with 14 to 30 KB of HTML initial HTML document size have a good INP most often: 98%, against 97% of all WordPress sites. Sites with over 75 KB of HTML HTML size have it least often, at 95%.

14 to 30 KB of HTML is the most used: 39.6% of WordPress sites. Under 14 KB of HTML is 11.8% of WordPress sites.

Across the four LCP sub-parts in Chrome's field data, the largest gap between under 14 KB of HTML and over 75 KB of HTML sits in time to first byte (TTFB): 917 ms median for under 14 KB of HTML against 1352 ms for over 75 KB of HTML. Overall median field LCP is 1.7s for under 14 KB of HTML (TTFB 917 ms, load delay 293 ms, load duration 169 ms, render delay 217 ms) versus 2.2s for over 75 KB of HTML.

Across the four LCP sub-parts in Chrome's field data, the largest gap between under 14 KB of HTML and over 75 KB of HTML sits in time to first byte (TTFB): 917 ms median for under 14 KB of HTML against 1352 ms for over 75 KB of HTML. Overall median field LCP is 1.7s for under 14 KB of HTML (TTFB 917 ms, load delay 293 ms, load duration 169 ms, render delay 217 ms) versus 2.2s for over 75 KB of HTML.

On jQuery, 40% of sites use 14 to 30 KB of HTML initial HTML document size, and 69% of jQuery sites have a good LCP, against 70% of all sites. On Vue, 44% of sites use 30 to 75 KB of HTML, and 67% of Vue sites have a good LCP.

On jQuery, 40% of sites use 14 to 30 KB of HTML initial HTML document size, and 97% of jQuery sites have a good INP, against 97% of all sites. On Vue, 44% of sites use 30 to 75 KB of HTML, and 95% of Vue sites have a good INP.

FrameworkInitial HTML document size on its sitesGood LCP

All WordPress sites 63,295 sites14 KB to 30 KB 40%30 KB to 75 KB 38%Under 14 KB (First TCP window) 12%Over 75 KB 11%70%

Alpine 380 sites30 KB to 75 KB 43%14 KB to 30 KB 38%Over 75 KB 13%78%

jQuery 60,314 sites14 KB to 30 KB 40%30 KB to 75 KB 38%Under 14 KB (First TCP window) 12%Over 75 KB 11%69%

Vue 4,844 sites30 KB to 75 KB 44%14 KB to 30 KB 33%Over 75 KB 18%67%

React 2,233 sites30 KB to 75 KB 54%Over 75 KB 20%14 KB to 30 KB 20%63%

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

Does the framework explain the initial HTML document size difference?

On jQuery, 81% of sites with under 14 KB of HTML initial HTML document size have a good LCP, against 72% of jQuery sites with 14 to 30 KB of HTML. Within jQuery, sites with under 14 KB of HTML have a good LCP more often. On Vue, 80% of sites with under 14 KB of HTML initial HTML document size have a good LCP, against 71% of Vue sites with 14 to 30 KB of HTML. Within Vue, sites with under 14 KB of HTML and 14 to 30 KB of HTML have a good LCP about equally often.

On jQuery, 98% of sites with 14 to 30 KB of HTML initial HTML document size have a good INP, against 96% of jQuery sites with 30 to 75 KB of HTML. Within jQuery, sites with 14 to 30 KB of HTML have a good INP more often. On Vue, 98% of sites with 14 to 30 KB of HTML initial HTML document size have a good INP, against 95% of Vue sites with 30 to 75 KB of HTML. Within Vue, sites with 14 to 30 KB of HTML have a good INP more 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 7,455 sites (11.8%) whose HTML fits under 14 KB, 82% achieve good LCP (1.6s median, 917 ms TTFB). Sites between 14 and 30 KB (39.6%) record 73% good LCP (1.9s median), while pages bloating to 30 to 75 KB (37.8%) drop to 65% good LCP (2.1s median) and pages over 75 KB (10.9%) sit at 59% good LCP (2.2s median).