In this guide
Core Web Vitals are Google's three measurements of how a page feels to real visitors: how fast the main content appears, how quickly the page reacts to a tap or click, and whether things jump around while it loads. They are part of Google's page experience signals, and more importantly they decide whether visitors stay.
Key takeaways
- Core Web Vitals are three measurements of real visits: loading (LCP), responsiveness (INP) and visual stability (CLS).
- Good means LCP within 2.5 s, INP within 200 ms and CLS at most 0.1, for at least 75% of visits.
- Lab tests (Lighthouse) explain problems; field data from real Chrome users is what Google uses.
- Large images, slow servers, heavy JavaScript and ads or banners without reserved space cause most failures.
The three measurements
Google looks at the 75th percentile of real visits: a page passes when at least three in four visits are good.
- Largest Contentful Paint (LCP): how long until the biggest image or text block in view is shown. Good: 2.5 seconds or less; poor: over 4 seconds.
- Interaction to Next Paint (INP): how long the page takes to respond visibly after a tap, click or key press. Good: 200 milliseconds or less; poor: over 500.
- Cumulative Layout Shift (CLS): how much visible content moves unexpectedly while the page loads. Good: 0.1 or less; poor: over 0.25.
Lab data and field data
PageSpeed Insights shows two kinds of numbers. Lab data comes from one simulated test run (Lighthouse) on a mid-range phone and network; it is repeatable and explains what is slow. Field data comes from real Chrome users over the last 28 days, and only exists when a page has enough visits.
INP can only be measured with real visitors, so lab tests show Total Blocking Time instead: a good stand-in, because long JavaScript tasks are what make pages slow to respond.
What usually makes LCP slow
- A large hero image that is not compressed, not sized for phones, or lazy-loaded.
- A slow server response (time to first byte), often from shared hosting or no caching.
- Render-blocking CSS and fonts that must download before anything is drawn.
- Sliders and page builders that load their content with JavaScript.
What usually breaks INP and CLS
- INP: heavy JavaScript from page builders, chat widgets, tracking tags and third-party embeds that keep the browser busy.
- CLS: images and iframes without width and height, cookie banners and ads that push content down, and web fonts that swap in at a different size.
Fix the biggest problem first
- Run PageSpeed Insights on mobile for your most visited page.
- Read the Diagnostics for the largest item: it names the image, script or font to fix.
- Fix that one item, publish, and test again.
- Repeat for the next page template (blog post, product page), since pages built the same way share the same problems.
Checklist
- Test your home page and your busiest page on mobile in PageSpeed Insights.
- Compress and correctly size the largest image above the fold; do not lazy-load it.
- Give images, embeds and ad slots a fixed width and height.
- Remove or delay scripts you do not need on every page.
- Re-test weekly so a new plugin or theme update does not slip through.
Questions
Do Core Web Vitals affect Google rankings?
They are one of many signals and rarely outweigh relevant, useful content. They matter more for visitors: slow, jumpy pages lose people before they read anything.
Why does my score change every time I test?
Lab tests vary with network and server load at that moment. Look at the trend over several tests, or at field data, rather than one run.
My site scores 100 on desktop but 50 on mobile. Which matters?
Mobile. Google indexes the mobile version of pages, and the mobile test uses a slower phone and network, which is closer to many real visitors.