The State of Web Vitals

HTTP Last-Modified headers on React sites

99.7% of audited sites (9,537) include Last-Modified timestamp validators (85% good LCP, 1.4s median), while 0.3% omit them (76% good LCP, 1.9s median).

Field data PhoneDesktopAll Scope All sites October 2026 edition · All devices 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 85% of those sites have a good LCP, against 85% of all React sites.

The State of Web Vitals · October 2026 · React: 9,570 crawled sites · 9,110 with all devices 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: 85%, against 85% of all React sites.

SitesGood, needs improvement, poor LCPMedian
All React sites9,11085%1.4s
Last-Modified present 99.7%9,07785%1.4s
No Last-Modified 0.3%3376%1.9s
GoodNeeds improvementPoorSites: with Chrome field data
The State of Web Vitals · October 2026 · React: 9,570 crawled sites · 9,110 with all devices 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 · 9,110 with all devices 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): 515 ms median for Last-Modified headers against 925 ms for no Last-Modified header. Overall median field LCP is 1.4s for Last-Modified headers (TTFB 515 ms, load delay 439 ms, load duration 146 ms, render delay 174 ms) versus 1.9s for no Last-Modified header.

Median LCP phase breakdown (Chrome field p75) Load delay Median LCP
All React sites 6,740516 ms439 ms439 mslab 439 ms1.4s
Last-Modified present 6,715515 ms439 ms439 mslab 439 ms1.4s
TTFB Resource load delay Resource load time Element render delay
The State of Web Vitals · October 2026 · React: 9,570 crawled sites · 9,110 with all devices 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 94% of Wix sites have a good LCP, against 85% of all sites.

PlatformLast-Modified cache validation on its sitesGood LCP
All React sites 9,570 sitesLast-Modified present 100%85%
Wix 5,824 sitesLast-Modified present 100%94%
WordPress 2,342 sitesLast-Modified present 99%61%
Last-Modified present No Last-Modified
The State of Web Vitals · October 2026 · React: 9,570 crawled sites · 9,110 with all devices 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 85% good LCP (1.4s median, 515 ms TTFB), compared with 76% good LCP (1.9s median, 925 ms TTFB) on the 0.3% of sites that omit it.

The State of Web Vitals · October 2026 · React: 9,570 crawled sites · 9,110 with all devices 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 (85% good LCP, 1.4s median) and 0.3% omit it (76% good LCP, 1.9s median).

Go fix it TTFB: cache duration →
The State of Web Vitals · October 2026 · React: 9,570 crawled sites · 9,110 with all devices 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 (9,110 with all devices Chrome field data, October 2026) · How we measured