Core Web Vitals are three measurements Google uses to describe how a page feels to real visitors. You do not need to be a developer to understand them, and knowing them will help you judge the work of anyone who builds your site.
The three measurements
| Measurement | What it asks | Good score |
|---|---|---|
| Largest Contentful Paint (LCP) | How quickly does the main content appear? | 2.5 seconds or less |
| Interaction to Next Paint (INP) | How quickly does the page respond to a tap or click? | 200 milliseconds or less |
| Cumulative Layout Shift (CLS) | Does content jump around as the page loads? | 0.1 or less |
Google assesses these using data from real visitors, and looks at the experience of the 75th percentile. In other words, most of your visitors should be getting a good experience, not only those with the fastest phones.
Why they matter
There are two reasons, and the second is often the bigger one.
Search. Page experience is one of many signals Google considers. A fast page will not outrank a far more relevant one, but when pages are otherwise similar, a poor experience will not help you.
Customers. Someone on mobile data who is waiting for your page may simply go back and tap the next result. A layout that jumps can make them tap the wrong thing. Neither of these shows up as a complaint. You simply never hear from them.
How to check your site
Two free Google tools are enough to start:
- PageSpeed Insights shows real-visitor data, if your site has enough traffic, along with lab test results.
- Search Console shows which groups of pages on your site need attention.
Always look at the mobile results first, because they are usually the harder ones to pass.
The usual culprits
- Oversized images. A photo straight from a camera can be many times larger than it needs to be.
- Too many plugins and scripts. Every chat widget, tracker and slider adds work for the phone to do.
- Slow hosting. If the server is slow to respond, everything else starts late.
- Images and ads without reserved space. When they load, they push the content down, and this causes layout shift.
What to ask your developer
Ask for the mobile scores before and after any website project, measured the same way. If speed is promised, it should be possible to show it.