The State of Web Vitals

HTTP Last-Modified headers on React sites

99.7% of audited sites (9,537) include Last-Modified timestamp validators (79% good LCP, 1.5s median), while 0.3% omit them (100% good LCP, 1.7s median).

Field data PhoneDesktopAll Scope All sites October 2026 edition · Desktop field outcomes · React sites only
React Scoped to 9,570 sites out of 210,902 analyzed (frameworks) Compare with all sites →

0.3% of React sites have no Last-Modified cache validation. Of the 99.7% that do, 100% use Last-Modified headers, and 79% of those sites have a good LCP, against 79% of all React sites.

The State of Web Vitals · October 2026 · React: 9,570 crawled sites · 1,979 with desktop field dataBy Arjen Karel · corewebvitals.io/state-of-cwv
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LCP pass rate

Sites with Last-Modified headers Last-Modified cache validation have a good LCP most often: 79%, against 79% of all React sites.

SitesGood, needs improvement, poor LCPMedian
All React sites1,97979%1.5s
No Last-Modified 0.3%13100%1.7s
Last-Modified present 99.7%1,96679%1.5s
GoodNeeds improvementPoorSites: with Chrome field data
The State of Web Vitals · October 2026 · React: 9,570 crawled sites · 1,979 with desktop field dataBy Arjen Karel · corewebvitals.io/state-of-cwv
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Last-Modified cache validation usage

99.7% of React sites use Last-Modified cache validation, and 100% of those use Last-Modified headers.

All React sites 9,570 sites
Last-Modified cache validation 99.7%
Sites with Last-Modified cache validation 9,537 sites
Last-Modified present 100.0%
Last-Modified present 100.0%
The State of Web Vitals · October 2026 · React: 9,570 crawled sites · 1,979 with desktop field dataBy Arjen Karel · corewebvitals.io/state-of-cwv
3

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): 707 ms median for Last-Modified headers against 475 ms for no Last-Modified header. Overall median field LCP is 1.6s for Last-Modified headers (TTFB 707 ms, load delay 413 ms, load duration 142 ms, render delay 192 ms) versus 1.6s for no Last-Modified header.

Median LCP phase breakdown (Chrome field p75) Load delay Median LCP
All React sites 1,324703 ms413 ms192 ms413 mslab 429 ms1.6s
Last-Modified present 1,314707 ms413 ms192 ms413 mslab 428 ms1.6s
TTFB Resource load delay Resource load time Element render delay
The State of Web Vitals · October 2026 · React: 9,570 crawled sites · 1,979 with desktop field dataBy Arjen Karel · corewebvitals.io/state-of-cwv
4

Last-Modified cache validation per platform

Wix sets the Last-Modified cache validation to Last-Modified headers on 100% of its sites, and 92% of Wix sites have a good LCP, against 79% of all sites.

PlatformLast-Modified cache validation on its sitesGood LCP
All React sites 9,570 sitesLast-Modified present 100%79%
Wix 5,824 sitesLast-Modified present 100%92%
WordPress 2,342 sitesLast-Modified present 99%66%
Last-Modified present No Last-Modified
The State of Web Vitals · October 2026 · React: 9,570 crawled sites · 1,979 with desktop field dataBy Arjen Karel · corewebvitals.io/state-of-cwv
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Last-Modified headers or the platform

How Last-Modified supports conditional requests and heuristic browser caching

Last-Modified is HTTP's original timestamp validator. Across 9,537 audited sites (99.7%), 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 79% good LCP (1.5s median, 707 ms TTFB), compared with 100% good LCP (1.7s median, 475 ms TTFB) on the 0.3% of sites that omit it.

The State of Web Vitals · October 2026 · React: 9,570 crawled sites · 1,979 with desktop field dataBy Arjen Karel · corewebvitals.io/state-of-cwv
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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, 99.7% of sites send Last-Modified (79% good LCP, 1.5s median) and 0.3% omit it (100% good LCP, 1.7s median).

Go fix it TTFB: cache duration →
The State of Web Vitals · October 2026 · React: 9,570 crawled sites · 1,979 with desktop field dataBy Arjen Karel · corewebvitals.io/state-of-cwv
Related signals HTTP protocol → HTML size (kB) → Connection hints → HTML compression → By Arjen Karel · 9,570 crawled React sites (1,979 with desktop Chrome field data, October 2026) · How we measured