Core Web Vitals is a name that sounds more complicated than the thing itself. Google identified three specific ways that a slow or unstable page irritates visitors, gave each one a name and a threshold, and incorporated them into how it evaluates site quality. That's the whole concept.

Understanding what each one measures tells you something useful: whether a problem is about how quickly content appears, whether the page responds when someone taps something, or whether the layout is jumping around while it loads. These are different problems with different causes and different fixes.

The three measurements

Largest Contentful Paint
LCP
Good: under 2.5s Needs work: 2.5-4s Poor: over 4s
LCP measures how long it takes for the largest visible element on the page to fully load. That's usually a hero image, a large heading, or a prominent block of text. It's the most direct measure of when a visitor actually sees something meaningful rather than staring at a blank or half-loaded screen. Of the three Core Web Vitals, this is the one that most directly reflects what a visitor experiences.
Interaction to Next Paint
INP
Good: under 200ms Needs work: 200-500ms Poor: over 500ms
INP measures the delay between a visitor doing something (tapping a button, clicking a menu item, filling in a form field) and the page visibly responding. A high INP score means the page feels laggy and unresponsive. For businesses with booking forms, product filters, or interactive elements, this directly affects whether people complete the actions you need them to complete. INP replaced the older FID metric in March 2024.
Cumulative Layout Shift
CLS
Good: under 0.1 Needs work: 0.1-0.25 Poor: over 0.25
CLS measures how much the page layout shifts while it's loading. You know the experience: you're about to tap a button and the page jumps, and you end up tapping the wrong thing. Or text reflows as an image loads in above it and you lose your place. A high CLS score reflects a visually unstable page. For mobile visitors on a slow connection, this is one of the most frustrating experiences a website can create.

Why they matter beyond the ranking signal

Google incorporated Core Web Vitals into its ranking algorithm in 2021. Pages that pass all three thresholds get a modest ranking benefit over equivalent pages that don't. But the more direct impact is on visitor behaviour, not rankings.

Pages that pass Core Web Vitals see a 24% reduction in bounce rates compared to pages that fail. A visitor who arrives at a page that loads quickly, responds to their input, and doesn't jump around while loading is more likely to stay long enough to read it, contact you, or buy something. A visitor who arrives at a page that takes five seconds to show anything, stutters when they try to scroll, and shifts the layout three times while they're reading it will leave. These are behavioural signals that affect both your actual conversion rate and, indirectly, the signals Google uses to assess your site's quality.

What causes a bad CLS score: The most common culprit is images without declared dimensions. If the browser doesn't know how tall an image is before it loads, it can't reserve space for it — so when the image appears, everything below it shifts down. The fix is adding explicit width and height attributes to image tags. A developer can do this in under an hour across an entire site.

How to find your scores

Google Search Console shows your Core Web Vitals scores under the "Experience" section. This uses real-world data from Chrome users visiting your site, which makes it more accurate than lab tests. If you don't have Search Console set up, that's the first step — it's free, takes about 10 minutes to verify, and gives you data that no paid tool can replicate.

PageSpeed Insights (pagespeed.web.dev) gives you both lab data (simulated) and field data (real users) if your site has enough traffic. The free grader on this site uses the same underlying PageSpeed API and gives you the Performance score alongside your SEO, mobile, and security scores in a single report.

What typically causes a poor score

For LCP: uncompressed images are the most common issue, followed by slow hosting that takes too long to start sending any data. A hero image that's 3MB when it could be 150KB is often the entire problem.

For INP: too much JavaScript executing on the main thread. This is usually third-party scripts — analytics, chat widgets, cookie consent tools, social sharing buttons. Each one adds work the browser has to complete before it can respond to user input.

For CLS: images without dimensions, web fonts that swap in after the page has already rendered, and ads or embeds loading in without reserved space are the main causes.

The full audit identifies which of these is affecting your specific site and, more usefully, which ones to fix first. Not all Core Web Vitals failures have the same impact, and working through them in the right order makes the improvement quicker and cheaper.