WordPress HTTP Last-Modified headers and the Core Web Vitals
98.9% of audited sites (62,572) include Last-Modified timestamp validators (70% good LCP, 1.9s median), while 1.1% omit them (78% good LCP, 1.7s median).
1.1% of WordPress sites have no Last-Modified cache validation. Of the 98.9% that do, 100% use Last-Modified headers, and 70% of those sites have a good LCP, against 70% of all WordPress sites.
LCP pass rate
Sites with Last-Modified headers Last-Modified cache validation have a good LCP most often: 70%, against 70% of all WordPress sites.
Last-Modified cache validation usage
98.9% of WordPress sites use 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): 1129 ms median for Last-Modified headers against 1026 ms for no Last-Modified header. Overall median field LCP is 1.9s for Last-Modified headers (TTFB 1129 ms, load delay 340 ms, load duration 166 ms, render delay 236 ms) versus 1.8s for no Last-Modified header.
Last-Modified cache validation per framework
React sets the Last-Modified cache validation to Last-Modified headers on 99% of its sites, and 61% of React sites have a good LCP, against 70% of all sites.
Alpine 380 sitesLast-Modified present 99%80%
jQuery 60,314 sitesLast-Modified present 99%69%
Vue 4,844 sitesLast-Modified present 98%68%
React 2,233 sitesLast-Modified present 99%61%Last-Modified headers or the framework
Does the framework explain the Last-Modified cache validation difference?
Almost every site with Last-Modified headers Last-Modified cache validation is a jQuery site, and almost every jQuery site has Last-Modified headers Last-Modified cache validation, so the data can't tell Last-Modified headers Last-Modified cache validation and jQuery apart. On Vue, 68% of sites with Last-Modified headers Last-Modified cache validation have a good LCP, against 65% of Vue sites with no Last-Modified header. Within Vue, sites with Last-Modified headers and no Last-Modified header have a good LCP about equally often.
How Last-Modified supports conditional requests and heuristic browser caching
Last-Modified is HTTP's original timestamp validator. Across 62,572 audited sites (98.9%), 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 70% good LCP (1.9s median, 1129 ms TTFB), compared with 78% good LCP (1.7s median, 1026 ms TTFB) on the 1.1% of sites that omit it.
Understanding HTTP Last-Modified headers
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, 98.9% of sites send Last-Modified (70% good LCP, 1.9s median) and 1.1% omit it (78% good LCP, 1.7s median).
By Arjen Karel · 63,295 crawled WordPress sites (60,855 with all devices Chrome field data, October 2026). How we measured.