WordPress HTML compression (Brotli vs Gzip) and the Core Web Vitals

Sites with Brotli HTML compression have a good LCP most often: 78%, against 70% of all WordPress sites. Sites with Gzip HTML compression have it least often, at 66%.

Nine in ten WordPress sites uses HTML compression, and 57% of those use Gzip. Brotli is 36% of the sites with HTML compression, 31.4% of all WordPress sites.

Across the four LCP sub-parts in Chrome's field data, the largest gap between Brotli and no sits in time to first byte (TTFB): 933 ms median for Brotli against 1244 ms for no. Overall median field LCP is 1.8s for Brotli (TTFB 933 ms, load delay 343 ms, load duration 193 ms, render delay 236 ms) versus 2.1s for no.

On jQuery, 51% of sites use Gzip HTML compression, and 69% of jQuery sites have a good LCP, against 70% of all sites. On Vue, 56% of sites use Gzip, and 67% of Vue sites have a good LCP.

FrameworkHTML compression on its sitesGood LCP

WordPress sites with HTML compression 63,291 sitesGzip 50%Brotli (br) 31%Uncompressed (none) 12%70%

Alpine 380 sitesBrotli (br) 40%Gzip 38%Zstandard (zstd) 13%78%

jQuery 60,314 sitesGzip 51%Brotli (br) 31%Uncompressed (none) 12%69%

Vue 4,844 sitesGzip 56%Brotli (br) 28%Uncompressed (none) 13%67%

React 2,233 sitesGzip 48%Brotli (br) 35%63%

Gzip Brotli (br) Uncompressed (none) Zstandard (zstd)

Does the framework explain the HTML compression difference?

On jQuery, 66% of sites with Gzip HTML compression have a good LCP, against 63% of jQuery sites without HTML compression. Within jQuery, sites with Gzip HTML compression have a good LCP more often. On Vue, 65% of sites with Gzip HTML compression have a good LCP, against 60% of Vue sites without HTML compression. Within Vue, sites with and without HTML compression have a good LCP about equally often.

Why Brotli HTML compression speeds up resource discovery

Brotli uses a built-in dictionary of common HTML, CSS, and JavaScript tokens, shrinking HTML documents 15 to 20 percent more than Gzip at comparable CPU cost. Because the browser's preload scanner parses the HTML stream chunk by chunk as it arrives, a smaller compressed document delivers the <head> and hero <img> markup in fewer TCP or QUIC packets. Sites serving Brotli HTML record a 933 ms median TTFB and 1.8s median LCP, compared with 1231 ms TTFB and 2.0s LCP on Gzip.