What are Core Web Vitals?
Core Web Vitals are three Google metrics measuring real-user page experience: Largest Contentful Paint for loading, Interaction to Next Paint for responsiveness, and Cumulative Layout Shift for visual stability. They are ranking signals and, more importantly, direct predictors of whether visitors stay.
Understanding Core Web Vitals
The thresholds are specific. LCP should complete within 2.5 seconds, INP should stay under 200 milliseconds, and CLS should stay below 0.1 — each measured at the 75th percentile of real visits, not in a lab. That percentile matters: a site that is fast on a developer's machine and slow on a mid-range Android on mobile data is failing for most of its actual audience.
As a ranking factor the weight is modest and easy to overstate — content relevance dominates, and a fast irrelevant page does not outrank a slow relevant one. As a conversion factor the weight is large and consistently measurable, which is the better reason to care. Every additional second before a page becomes usable costs a measurable share of visitors on mobile.
The most common real-world causes are unremarkable: unoptimised hero images and render-blocking resources for LCP, heavy JavaScript on the main thread for INP, and images or embeds without reserved space for CLS. Field data from the Chrome UX Report is the source of truth; a lab tool like Lighthouse is useful for diagnosis but does not reflect what your users experienced.
Key points
- LCP under 2.5s, INP under 200ms, CLS under 0.1
- Measured at the 75th percentile of real visits, not in a lab
- Modest as a ranking signal, substantial as a conversion factor
- Common causes: unoptimised images, heavy JS, unreserved layout space
- Field data is authoritative; Lighthouse is a diagnostic
Where this gets oversold
A perfect Lighthouse score means very little on its own. It is a lab simulation on one synthetic connection; field data from real visitors regularly tells a completely different story, and the field data is the one Google uses.
Related terms
Core Web Vitals — related questions
What is a good Core Web Vitals score?
Passing means LCP under 2.5 seconds, INP under 200 milliseconds, and CLS under 0.1, each at the 75th percentile of real user visits. All three must pass for a page to be classed as good.
How much do Core Web Vitals affect rankings?
Modestly. They act as a tiebreaker between pages of comparable relevance rather than a primary factor. The stronger business case is conversion — slow pages lose visitors on mobile at a rate that is easy to measure directly.
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