At a glance the headline numbers for Last-Modified present
Whether responses carry Last-Modified validators.
95.6% of sites send Last-Modified headers.
Last-Modified present sites that have it vs sites that don't
95.6% of sites send Last-Modified headers (90,712 of 94,910). With send: CLS 0.00, 87.8% pass. Without: CLS 0.00, 88.2% pass. computed
Why this matters for the Core Web Vitals, and where to start fixing it
Last-Modified is the older validator. The browser sends the timestamp back (If-Modified-Since) and the server answers 304 Not Modified when nothing changed, saving the re-download of an expired cache entry.
It is weaker than an ETag: one-second resolution, and deploys often touch timestamps without changing content, which breaks the match. But any validator beats none. A response with no validator and a short TTL is re-downloaded in full on every revisit.
How does this affect the Core Web Vitals?
Having it makes no measurable difference to the LCP: 83% pass with it, 86% without.
The gap is bigger on INP: 95% pass with it, 89% without.
Chrome field data from 94,910 sites, representing millions of real page loads. How we measured.