What Are Core Web Vitals? Measuring Real User Experience
Core Web Vitals are a set of Google metrics that measure loading speed, interactivity, and visual stability as experienced by real users.
Page quality is hard to capture in one number, so Google defined a focused set of user-centric metrics called Core Web Vitals. They measure how fast the main content loads, how quickly the page responds to input, and how stable the layout is. They influence search ranking and guide frontend work, and CI can track them per build.
The three metrics
- Largest Contentful Paint (LCP): when the main content appears.
- Interaction to Next Paint (INP): how responsive the page feels to input.
- Cumulative Layout Shift (CLS): how much the layout jumps unexpectedly.
Why these three
They map to three felt experiences: is it loading, can I interact, and is it stable. Together they capture the moments users notice most, which is why Google chose them as the core set.
Lab versus field
Field data reflects real users and is what affects search ranking. Lab measurements, like those Lighthouse produces, are reproducible and good for catching regressions before release, but they are an approximation.
Common causes of poor vitals
Large unoptimized images hurt LCP, heavy JavaScript hurts INP, and images or ads without reserved space hurt CLS. Many fixes are build-time: optimize assets, split code, and reserve layout space.
Core Web Vitals in CI/CD
CI can measure lab vitals against a built preview on each pull request and fail the build when a metric regresses past a budget. Catching a vitals regression in the pipeline is far cheaper than discovering it in field data after it has already affected users and ranking.
Key takeaways
- Core Web Vitals measure loading (LCP), interactivity (INP), and stability (CLS).
- Field data affects ranking; lab data catches regressions before release.
- CI can gate on lab vitals budgets to catch regressions per pull request.