Core Web Vitals
Definition
Core Web Vitals are three metrics Google uses to measure real-world page experience. LCP (Largest Contentful Paint) measures loading — how long until the largest visible element renders; target under 2.5 seconds. CLS (Cumulative Layout Shift) measures visual stability — how much content jumps while loading; target under 0.1. INP (Interaction to Next Paint) measures responsiveness — the delay from a user interaction to the next visual update; target under 200 milliseconds. INP replaced FID as a Core Web Vital in March 2024. All three are assessed at the 75th percentile of real visits, so a fast experience for most users still fails if the slowest quarter struggles.
What are the passing thresholds for Core Web Vitals?
LCP under 2.5 seconds, INP under 200 milliseconds, CLS under 0.1. Each is measured at the 75th percentile of page loads across mobile and desktop separately, over a rolling 28-day window. A URL passes only when all three are within threshold. The 75th-percentile rule is the part teams miss: optimising the median changes nothing if the tail is where the failures live.
How much do Core Web Vitals affect rankings?
They are a real but secondary signal. Google has described page experience as a tiebreaker between pages of comparable relevance rather than a primary factor — content that answers the query still outranks a faster page that does not. The practical implication is to fix the three metrics to green and stop there; polishing LCP from 2.4s to 1.8s buys user experience, not position.
What is the difference between field data and lab data?
Field data comes from the Chrome User Experience Report — actual Chrome users on real devices and networks — and is what Google uses for assessment. Lab data comes from a simulated load in a tool like Lighthouse. Lab data is reproducible and useful for debugging; field data is the score that counts. They routinely disagree, and when they do, field data wins.
Why did INP replace FID?
First Input Delay only measured the delay before the browser began processing the first interaction — not how long the response actually took, and not any interaction after the first. Most pages passed it trivially, which made it a weak signal. INP measures the full duration from interaction to the next paint across the whole visit, so it reflects sluggishness that FID scored as fine.
What usually causes each metric to fail?
LCP is typically a slow server response, a render-blocking stylesheet, or a hero image that is lazy-loaded or unoptimised. INP is usually long JavaScript tasks blocking the main thread, often from third-party tags. CLS is almost always images and embeds without width and height, web fonts swapping in, or content injected above what the user is already reading.
Related Tools
Run a free SEO audit to see how your site performs.
Try the SEO Checker