Core Web Vitals are three metrics Google uses to judge how a web page feels to real visitors: how fast it loads, how quickly it responds to input and how visually stable it is. The metrics are called LCP, INP and CLS, and they affect both your Google rankings and how many visitors stay on your site and convert.
The three Core Web Vitals metrics
Google picked three metrics that together capture the perceived quality of a page. Each one has a clear threshold for a passing score.
LCP: Largest Contentful Paint
LCP measures how long it takes for the largest visible element on the page to load. That is usually a hero image, a video or a large block of text. LCP is the closest thing we have to measuring "when does the page feel ready".
Threshold: under 2.5 seconds is good. Between 2.5 and 4 seconds needs improvement. Over 4 seconds is poor.
INP: Interaction to Next Paint
INP measures how quickly the page responds when a visitor interacts with it: clicking a button, opening a menu or typing in a field. It looks at all interactions during a visit and reports one of the slowest. INP replaced the older FID metric in March 2024 and is considerably stricter, because it measures the full response time rather than just the delay before the page starts processing the click.
Threshold: under 200 milliseconds is good. Between 200 and 500 milliseconds needs improvement. Over 500 milliseconds is poor.
CLS: Cumulative Layout Shift
CLS measures visual stability: how much the content jumps around while the page loads. Everyone has experienced it. You go to tap a link, an ad loads in above it and you tap the wrong thing. CLS puts a number on that problem.
Threshold: under 0.1 is good. Between 0.1 and 0.25 needs improvement. Over 0.25 is poor.
How to measure Core Web Vitals
You do not need paid tools to measure. Google provides everything.
- PageSpeed Insights tests a single URL and shows both real user data and a lab test with concrete improvement suggestions. Start here.
- Google Search Console has a report under "Page experience" that shows how your whole site performs, split by mobile and desktop. This is where you see which page groups drag down the total.
- CrUX (Chrome User Experience Report) is the source behind both: anonymized data from real Chrome users. This is the data Google actually uses for ranking.
One important distinction: lab data (like Lighthouse) shows how a page performs in a simulated test. Field data (CrUX) shows how it performs for your real visitors. Field data is what counts. A page can score 95 in Lighthouse and still fail the thresholds in the real world, for example because many of your visitors use slow phones.
Why Core Web Vitals affect rankings and conversions
Core Web Vitals are a confirmed ranking factor in Google, but set your expectations right. They are one signal among many, and great content beats fast loading. In practice, the metrics work as a tiebreaker: when two pages are roughly equally relevant, the one with the better page experience often wins.
The bigger effect is commercial. Studies from Google and others show that the probability of a bounce rises sharply with every extra second of load time. A page that loads in 1.5 seconds instead of 4 keeps more visitors, gets more page views per session and converts better. In other words, you are not primarily optimizing for Google's algorithm. You are optimizing for your own revenue.
The most common causes of failing scores and how to fix them
Most Core Web Vitals problems come down to a handful of recurring causes.
Poor LCP
- Large, unoptimized images. Convert to WebP or AVIF, scale images to the size they are displayed at and prioritize the hero image with
fetchpriority="high". - A slow server. A high TTFB (time to first byte) eats the whole budget before the page even starts rendering. Use caching and a CDN.
- Render-blocking CSS and JavaScript. Inline the critical CSS and defer the rest.
Poor INP
- Heavy JavaScript tasks that block the main thread. Break up long tasks, remove scripts you do not use and audit your third-party scripts. Tag managers, chat widgets and analytics tools are often worse offenders than your own code.
- Too much work on every click. Render less, debounce events and move heavy work to web workers.
Poor CLS
- Images and ads without reserved space. Always set
widthandheighton images and reserve a fixed height for ad slots. - Web fonts that change appearance on load. Use
font-display: swaptogether with size-adjusted fallback fonts. - Content injected above existing content, such as banners and cookie notices. Position them so they do not push the page down.
Getting started: measure first, then fix
Start by looking up your site in Search Console and testing your most important pages in PageSpeed Insights. Fix the biggest problems first, usually images and JavaScript weight, and measure again after a couple of weeks since field data lags behind.
Core Web Vitals are one part of technical SEO as a whole. To understand the full picture, read our technical SEO guide. If you would rather get a concrete list of exactly what is slowing your site down, we do that for you in an SEO Audit Sprint, where Core Web Vitals is one of the areas we cover.
