If you’ve ever run your website through Google’s PageSpeed Insights, you’ve probably seen three unfamiliar abbreviations: LCP, INP and CLS. Together, they’re called Core Web Vitals, and they’re Google’s way of measuring how your website feels to real people.
They sound technical, but the ideas behind them are simple. This guide explains what they measure, why they matter to your business and how to check your own site.
Why Google measures “feel”
For years, website speed was measured with technical numbers that didn’t always match what people experienced. A page could be “loaded” by one measure while visitors were still staring at a blank screen.
Core Web Vitals were introduced to fix that. They focus on three moments every visitor notices: when the main content appears, how quickly the page responds when you tap something, and whether things jump around while you’re trying to read. Google made them part of its page experience signals in 2021.
The three Core Web Vitals
Largest Contentful Paint (LCP): “Can I see it yet?”
LCP measures how long it takes for the biggest piece of content on the screen, usually a main image or headline, to appear. It’s the moment a visitor feels the page has arrived.
Google’s target: 2.5 seconds or less.
Common causes of a poor score: large, uncompressed images; slow hosting; heavy themes or page builders; and too many scripts loading before the main content.
Interaction to Next Paint (INP): “Does it respond when I tap?”
INP measures how quickly the page reacts when someone clicks a button, opens a menu or types into a form. If you’ve ever tapped a button and nothing seemed to happen for a moment, that’s what INP captures. It replaced an older measure, First Input Delay, in March 2024.
Google’s target: 200 milliseconds or less.
Common causes of a poor score: too much JavaScript, often from plugins, chat widgets, tracking tools and page builders, keeping the browser busy.
Cumulative Layout Shift (CLS): “Stop moving!”
CLS measures how much the page jumps around while loading. You know the feeling: you go to tap a link, an image or banner loads above it, everything shifts and you tap the wrong thing.
Google’s target: a score of 0.1 or less.
Common causes of a poor score: images without set dimensions, late-loading ads or banners, cookie notices that push content down, and web fonts that swap in.
Why this matters to your business
These measurements line up closely with things that cost you money:
- Slow LCP means visitors give up before they see your offer. Google’s own research has shown bounce rates rising sharply as load times grow from one to several seconds.
- Poor INP makes your site feel broken. People who tap “Book now” and get no response often don’t try twice.
- High CLS causes mis-taps and frustration, and makes a site feel cheap and unreliable.
Core Web Vitals are also one of the signals Google considers when ranking pages. They won’t outweigh relevance and quality, but they can make a difference when competing pages are otherwise similar.
Lab data vs. real-world data
PageSpeed Insights shows two kinds of results, and the difference matters.
- Real-world data (labelled as what your real users are experiencing) comes from the Chrome User Experience Report: anonymous measurements from actual visitors over the previous 28 days. This is what Google uses for its assessment. Smaller sites may not have enough traffic to show it.
- Lab data is a one-off simulated test. It’s useful for diagnosing problems, but it’s a single run under fixed conditions.
If the two disagree, trust the real-world data for the big picture and use the lab data to find causes.
How to check your own site
- Visit pagespeed.web.dev and enter one of your key pages.
- Look at the mobile tab first.
- See whether your page “passes” the Core Web Vitals assessment, and which of the three metrics is weakest.
- If you use Google Search Console, open the Core Web Vitals report to see which groups of pages need attention across your whole site.
Check more than your homepage. Your service and contact pages are often where customers decide.
What usually fixes them
The fixes tend to be practical rather than exotic:
- Properly sized and compressed images, with dimensions set so the page doesn’t jump.
- Removing or delaying scripts that don’t need to load straight away.
- Fewer, leaner plugins, and lighter alternatives to heavy builders.
- Good caching and capable hosting.
- Loading fonts carefully so text appears quickly and doesn’t shift.
How long do improvements take to show?
Lab scores change as soon as you fix something, which is useful for confirming a change worked. Real-world data takes longer, because it’s a rolling 28-day window of actual visits. After a meaningful improvement, expect the real-world numbers in PageSpeed Insights and Search Console to move gradually over the following few weeks.
Search Console also lets you mark a group of issues as fixed and start a validation, which tracks whether the improvement holds. Be patient, keep measuring and avoid adding heavy new features in the meantime, or you may undo your own progress.
Don’t chase a perfect score
A score of 100 is satisfying, but it isn’t the goal. The goal is passing all three Core Web Vitals for real visitors on the pages that matter to your business. Some trade-offs, such as a genuinely useful booking widget or a video that sells your service, may be worth a few points. What matters is that every script and feature earns its place.
The takeaway
Core Web Vitals are Google’s way of measuring three things your customers already notice: whether your page appears quickly, responds when tapped and stays still while they read. Getting them right is good for visitors, good for conversions and helpful for search.
If your site doesn’t pass, or you’d like to know why, get in touch. Speed and Core Web Vitals are our specialty, and we’ll explain exactly what’s holding your pages back.