Unused CSS bytes on Wix sites

Sites with under 140 KB of unused CSS unused CSS bytes have a good LCP most often: 99%, against 92% of all Wix sites. Sites with 300 to 600 KB of unused CSS unused CSS have it least often, at 87%.

Under 140 KB of unused CSS is the most used: 33.8% of Wix sites.

Across the four LCP sub-parts in Chrome's field data, the largest gap between under 140 KB of unused CSS and 300 to 600 KB of unused CSS sits in time to first byte (TTFB): 288 ms median for under 140 KB of unused CSS against 469 ms for 300 to 600 KB of unused CSS. Overall median field LCP is 1.0s for under 140 KB of unused CSS (TTFB 288 ms, load delay 421 ms, load duration 238 ms, render delay 142 ms) versus 1.3s for 300 to 600 KB of unused CSS.

On React, 34% of sites use under 140 KB of unused CSS unused CSS bytes, and 92% of React sites have a good LCP, against 92% of all sites.

FrameworkUnused CSS bytes on its sitesGood LCP

All Wix sites 5,621 sitesUnder 140 KB unused 34%140 to 300 KB unused 31%300 to 600 KB unused 24%Over 600 KB unused 11%92%

React 5,620 sitesUnder 140 KB unused 34%140 to 300 KB unused 31%300 to 600 KB unused 24%Over 600 KB unused 11%92%

Under 140 KB unused 140 to 300 KB unused 300 to 600 KB unused Over 600 KB unused

Does the framework explain the unused CSS bytes difference?

On React, 99% of sites with under 140 KB of unused CSS unused CSS bytes have a good LCP, against 95% of React sites with 140 to 300 KB of unused CSS. Within React, sites with under 140 KB of unused CSS and 140 to 300 KB of unused CSS have a good LCP about equally often.

Why unused CSS slows both network transfer and style calculation

Browsers must match every selector in the CSSOM against the DOM tree during layout and style recalculation. Across 1,901 sites (33.8%) with under 140 KB of unused CSS, 99% pass field LCP at a 1.0s median, whereas the 11.1% (625) over 600 KB drop to 82% good LCP (1.4s median).