CWV is measured in two ways: in the lab (Lighthouse, PageSpeed Insights synthetic runs) and in the field (Chrome User Experience Report, which logs anonymised performance from real Chrome users). The field data is what Google uses as a ranking input. Lab data is useful for debugging.
§01Definition
Definition
Core Web Vitals (CWV) is Google's measurement set for real-user page performance — currently Largest Contentful Paint (LCP), Interaction to Next Paint (INP), and Cumulative Layout Shift (CLS). INP replaced First Input Delay (FID) as a Core Web Vital on 12 March 2024.
The framework launched in May 2020 with three vitals: LCP, FID, CLS. In 2022 Google announced INP as an "experimental" Web Vital and ran it in parallel with FID. On 12 March 2024, INP officially replaced FID as a Core Web Vital. Sites that had not optimised for INP-style interaction responsiveness saw their CWV scores degrade overnight in the data Google's ranking systems consume.
§02The vitals
The three vitals and their thresholds
LCP measures loading — the time from page navigation to the render of the largest visible image or text block. Threshold: ≤ 2.5s good, ≤ 4.0s needs improvement, > 4.0s poor. INP measures interactivity — the time from a user interaction to the next visual update. Threshold: ≤ 200ms good, ≤ 500ms needs improvement, > 500ms poor. CLS measures visual stability — the sum of unexpected element-movement scores during page lifespan. Threshold: ≤ 0.1 good, ≤ 0.25 needs improvement, > 0.25 poor.
Two operational details. (1) The "good" threshold needs to hit on the 75th percentile of real-user samples for the URL or origin — not just on the median. A site whose median LCP is 2.0s but whose 75th-percentile is 3.4s is still in "needs improvement" territory. (2) The thresholds apply per device class (mobile vs desktop) separately. A site that passes on desktop and fails on mobile is rated as failing for the mobile-search experience.
§03Ranking signal
CWV as a ranking signal
Core Web Vitals is a ranking input but a small one. Google's documentation describes it as a tiebreaker that can lift a borderline page or hold one back, not as a primary determinant. The strongest practical effect is on competitive query spaces where multiple pages serve roughly equivalent content — CWV-passing pages tend to win against CWV-failing equivalents.
The honest framing: do not expect CWV optimisation to fix a relevance problem. A page that ranks #20 because the content is thin will not rank #5 because LCP dropped from 4s to 2s. CWV's value is at the margin — for the page that ranks #3 and could be #1 if Google had a reason to prefer it. For Squarespace owners with a default-template site3, CWV work is worth doing but not first.
§04Squarespace
Core Web Vitals on Squarespace specifically
Squarespace 7.1 ships with predictable CWV characteristics. LCP is usually the weakest vital because hero images are large and not always preloaded efficiently. CLS is moderate — Squarespace handles font-display and image sizing better than the average DIY site, but gallery blocks can still shift on load. INP is typically the strongest of the three because Squarespace's JavaScript bundle is well-optimised for interaction responsiveness on text-heavy pages.
The Squarespace-specific levers: (1) toggle the built-in image optimisation in Site Tools (it's not on by default for every site); (2) compress hero images before upload (Squarespace will not strip large file sizes for you); (3) avoid third-party widgets that inject blocking JavaScript (chat overlays, popup tools); (4) use Squarespace's native gallery blocks rather than custom-code carousels (the native versions are tuned for CLS); (5) audit per-page CWV in Search Console → Page Experience to identify pattern offenders. The full install treatment for each vital lives in LCP, INP, and CLS.
§05Related
Related terms
Core Web Vitals umbrellas three specific metrics. Each has its own glossary entry below.
