Core Web Vitals are three metrics Google uses to measure real-world page experience. They are part of Google's ranking systems, and — more importantly — they track what your visitors actually feel.
The thresholds
- Largest Contentful Paint (LCP) — how long until the main content is visible. Good: 2.5s or less. Poor: over 4s.
- Interaction to Next Paint (INP) — how quickly the page responds to taps and clicks. Good: 200ms or less. Poor: over 500ms.
- Cumulative Layout Shift (CLS) — how much content jumps around while loading. Good: 0.1 or less. Poor: over 0.25.
To pass, a page needs all three to be “good” at the 75th percentile of visits — meaning three out of four visitors get a good experience.
Field data vs. lab data
PageSpeed Insights shows two kinds of data. Field data comes from the Chrome User Experience Report (CrUX): real visitors over the last 28 days. Lab data is a single Lighthouse test on a simulated mid-range phone. Google uses field data for rankings; the Lighthouse performance score is a diagnostic tool.
That's why your score can be 55 while you pass Core Web Vitals — or 90 while you fail. And why fixes take up to four weeks to show up in field data.
What usually fails on WordPress
- LCP: lazy-loaded or oversized hero images, slow servers, render-blocking CSS. Start with this LCP fix.
- INP: heavy JavaScript from page builders and third-party widgets. See minimize main-thread work.
- CLS: images without dimensions, late-loading fonts, injected banners. See layout shifts from images.
