What the Core Web Vitals are
The Core Web Vitals are three measurements from Google for the experience on a page: how fast the main content appears, how quickly the page responds and how steady it stays while loading. Google takes them into account when ranking pages in search.
How it works
The three values and their limits for "good":
- LCP (Largest Contentful Paint): when the largest visible element appears. Good up to 2.5 seconds.
- INP (Interaction to Next Paint): how quickly the page responds to input. Good up to 200 ms.
- CLS (Cumulative Layout Shift): how much content shifts around. Good up to 0.1.
The 75th percentile is assessed: three out of four page views must meet the limit.
There are two kinds of measurement. Field data comes from real visitors using Chrome and is collected in the Chrome User Experience Report over the last 28 days; it only exists for pages with enough visits. Lab data is measured by Lighthouse, which loads the page once in a simulated browser, for mobile with a throttled device and network. This produces the Lighthouse score from 0 to 100: 90 and above is good, 50 to 89 needs improvement, anything below is poor.
bytereport gets the values from Google PageSpeed Insights if the operator of the instance has switched this on; the checked address is then sent to Google. Field data takes precedence, lab data steps in otherwise. The lab has no input, so Total Blocking Time stands in for INP there (good up to 200 ms, poor over 600 ms).
On top comes First Contentful Paint (FCP): when the very first content appears. Up to 1.8 seconds is good, over 3 seconds counts as poor. It is not one of the Core Web Vitals, but it shows whether things are stuck right at the start.
What matters
- Field data counts. Google Search uses the values of real visitors, not the Lighthouse score. The score helps you find causes.
- Mobile and desktop separately. Both are recorded separately. bytereport checks the mobile view by default; you choose the desktop view when you start the check.
- Patience. Field data covers 28 days; an improvement shows up gradually.
- Fluctuation. The Lighthouse score varies by a few points between two measurements.
- Not everything. Good values alone don't lift your position in search. Google names page experience as one of many signals; content and relevance come first.
How to improve them
The report covers the most common causes: work through the errors and warnings from the top. In Google Search Console, the "Core Web Vitals" report shows which pages of your site are affected. PageSpeed Insights shows field and lab data with diagnostics for a single page. The work itself is usually done by your agency or whoever built the site.