HTTP ETag cache validators on React sites

Sites with ETag validators ETag cache validation have a good LCP most often: 85%, against 85% of all React sites.

97.1% of React sites use ETag cache validation, and 100% of those use ETag validators.

Across the four LCP sub-parts in Chrome's field data, the largest gap between ETag validators and no ETag header sits in time to first byte (TTFB): 505 ms median for ETag validators against 1098 ms for no ETag header. Overall median field LCP is 1.4s for ETag validators (TTFB 505 ms, load delay 442 ms, load duration 146 ms, render delay 172 ms) versus 2.0s for no ETag header.

Wix sets the ETag cache validation to ETag validators on 100% of its sites, and 94% of Wix sites have a good LCP, against 85% of all sites.

PlatformETag cache validation on its sitesGood LCP

All React sites 9,570 sitesETag header present 97%85%

Wix 5,824 sitesETag header present 100%94%

WordPress 2,342 sitesETag header present 89%No ETag header 11%61%

ETag header present No ETag header

Does the platform explain the ETag cache validation difference?

On WordPress, 60% of sites with ETag validators ETag cache validation have a good LCP, against 68% of WordPress sites with no ETag header. Within WordPress, sites with ETag validators and no ETag header have a good LCP about equally often.

Why ETag headers matter on repeat visits and CDN edge revalidations

An Entity Tag (ETag) is an opaque version identifier attached to an HTTP response. Across 9,291 audited sites (97.1%) that emit ETag headers, browsers and CDN edges can send If-None-Match conditional requests when cached files expire and receive a header-only 304 Not Modified response instead of re-downloading the full payload. Because first-visit crawl and field LCP measurements weight initial loads and immutable hashed assets heavily, sites with ETag headers record 85% good LCP (1.4s median, 505 ms TTFB) while the 2.9% without ETag headers record 69% good LCP (1.9s median, 1098 ms TTFB).