Core Web Vitals are three measures Google uses to judge how your site treats a visitor. They feed into how your page is rated in search, but what matters more is that they measure something real: how long people wait, how much the interface lags, and whether content jumps under their finger.
This post explains each one without the jargon and says what drags them down.
Why they're measured on real visitors
An important difference many people miss: Google doesn't use the score from a testing tool, but data collected from real people who opened your site over the last 28 days.
That's why a site can score brilliantly in a test and badly in reality. The test runs on a fast connection, while your customers are on a phone at the edge of signal.
LCP: how long the main content takes
Largest Contentful Paint measures when the largest element on the first screen appears: usually a big image or the main heading.
- Good: under 2.5 seconds
- Needs improvement: 2.5 to 4 seconds
- Poor: over 4 seconds
This is the measure most closely tied to visitors leaving. Someone staring at a blank screen for four seconds goes back to the search results.
What usually drags it down: an oversized header image, a slow server, fonts that block rendering, too much code that has to run before the first image.
INP: how much the interface lags
Interaction to Next Paint measures how long it takes from your tap to the moment something on the screen changes. It replaced the older FID measure and is stricter.
- Good: under 200 milliseconds
- Needs improvement: 200 to 500 ms
- Poor: over 500 ms
It's that feeling when you tap a menu, nothing happens for a second, and you tap again.
What usually drags it down: too much JavaScript running at once, third-party scripts (chat widgets, tracking tools, ad systems), heavy plugins.
CLS: does the content jump
Cumulative Layout Shift measures how much content moves while the page loads.
- Good: under 0.1
- Needs improvement: 0.1 to 0.25
- Poor: over 0.25
It's when you go to tap a link, an image above loads at that moment and pushes everything down, and you tap something else.
What usually drags it down: images without set dimensions, ads injected later, cookie notices that push content, fonts that swap and change the size of the text.
How to check your site
PageSpeed Insights: enter the address. The top section shows real-visitor data if there's enough of it; that's what counts.
Search Console has a Core Web Vitals report that groups all your pages, so you can see whether the problem is one type of page or everywhere.
Lighthouse in Chrome for a quick check while you work. Its score isn't what Google uses, but it shows the direction well.
Always look at the mobile score. Most traffic comes from phones and the scores there are noticeably stricter.
How much it really affects rankings
Honestly: less than is sometimes claimed.
Content and links decide more. Speed is one signal and moves things when everything else is equal, which on competitive queries is often the case.
The bigger effect is direct and doesn't depend on Google: a faster site loses fewer people. You see it in the number of enquiries before you see it in rankings.
What to do first
In order of impact for the effort:
- Images. Modern formats, sizes matched to the device, set dimensions so nothing jumps. The biggest gain for the least work.
- Third-party scripts. List them and remove the ones you don't use. Every chat widget and tracking tool has its cost.
- Fonts. Limit how many you load and set them so they don't block rendering.
- Server and caching. If the response is slow, everything else is slow.
- The amount of JavaScript. The biggest job, and it usually touches how the site is built.
If your site is on a platform or on WordPress, the first three steps are doable. The fifth often isn't; more on that in why WordPress is slow.
Frequently asked questions
How soon will I see a change?
The data is calculated over 28 days, so expect the full change about a month after the fix.
Do all three have to be green?
For a good rating, yes, but don't chase perfection. The difference between poor and good matters; the difference between good and perfect rarely changes your business.
Why does the test show 95 while Search Console says it's poor?
The test runs on a fast connection; Search Console measures your real visitors.
Can it be fixed without a new site?
Often yes, at least partly. What decides speed is covered in what makes a website fast.
How to check where you stand
Run your site through PageSpeed Insights and look at the mobile score. If something is red, send me the address and I'll tell you whether it's a small fix or goes deeper.