HTTP Last-Modified headers and the Core Web Vitals
91.5% of audited sites (192,970) include Last-Modified timestamp validators (82% good LCP, 1.5s median), while 8.5% omit them (85% good LCP, 1.3s median).
One in ten sites, 8.5%, have no Last-Modified cache validation. Of the 91.5% that do, 100% use Last-Modified headers, and 82% of those sites have a good LCP, against 82% of all sites.
LCP pass rate
Sites with Last-Modified headers Last-Modified cache validation have a good LCP most often: 82%, against 82% of all sites.
Last-Modified cache validation usage
Nine in ten sites uses Last-Modified cache validation, and 100% of those use Last-Modified headers.
LCP phase breakdown for sites with and without Last-Modified headers
Across the four LCP sub-parts in Chrome's field data, the largest gap between Last-Modified headers and no Last-Modified header sits in time to first byte (TTFB): 652 ms median for Last-Modified headers against 565 ms for no Last-Modified header. Overall median field LCP is 1.5s for Last-Modified headers (TTFB 652 ms, load delay 321 ms, load duration 144 ms, render delay 225 ms) versus 1.4s for no Last-Modified header.
Understanding HTTP Last-Modified headers
How Last-Modified supports conditional requests and heuristic browser caching
Last-Modified is HTTP's original timestamp validator. Across 192,970 audited sites (91.5%), responses include Last-Modified so browsers can send If-Modified-Since on expired cache entries and RFC 9111 caches can apply heuristic freshness when explicit max-age directives are absent. Sites sending Last-Modified reach 82% good LCP (1.5s median, 652 ms TTFB), compared with 85% good LCP (1.3s median, 565 ms TTFB) on the 8.5% of sites that omit it.
Emit Last-Modified headers alongside ETag and Cache-Control on static and cacheable responses so both browsers and CDN edge nodes can revalidate stale objects via If-Modified-Since. In containerized CI/CD pipelines that rewrite file modification timestamps on every deploy, pair Last-Modified with content-hash ETag validators so unchanged files still return 304 Not Modified. Across the dataset, 91.5% of sites send Last-Modified (82% good LCP, 1.5s median) and 8.5% omit it (85% good LCP, 1.3s median).
How Last-Modified cache validation plays out per platform
What the field data shows when you isolate a single CMS or framework.
WordPress 65,509 sites 98.8% of WordPress sites send Last-Modified headers on responses.
Across WordPress sites, 98.8% emit Last-Modified headers (70% good LCP, 1.9s median).
React & Next.js 9,570 sites 99.7% of React & Next.js sites include Last-Modified headers on static and CDN responses.
Across React & Next.js deployments, 99.7% emit Last-Modified timestamps on static chunks or media assets (85% good LCP, 1.4s median).
By Arjen Karel · 210,902 crawled sites (196,263 with all devices Chrome field data, October 2026). How we measured.