Core Web Vitals

Core Web Vitals is Google's set of three standard user-experience metrics: LCP for perceived load speed, INP for responsiveness, and CLS for visual stability.

The value of these three is that instead of dozens of technical numbers they fold user experience into three legible dimensions with a shared verdict: each metric falls into a good, needs-improvement or poor band. Good means LCP up to 2.5 s, INP up to 200 ms and CLS up to 0.1. What often gets lost is that the verdict is taken at the 75th percentile of visits, not the average, and split by mobile and desktop, because an average hides the very slow users who should be seen. A second trap is that all three can be measured either in the lab (a synthetic run) or in the field (real users); the number you see in a developer tool on a strong device differs from what a real user experiences on a weak phone. Since March 2024, INP replaced the older FID as the third member.