Core Web Vitals explained: what LCP, INP and CLS mean for your business

If you’ve ever run your website through Google’s PageSpeed Insights, you’ve seen three acronyms near the top of the report: LCP, INP and CLS. These are Google’s Core Web Vitals, and they measure how your site feels to a real person using it.

They matter for two reasons. They’re part of how Google judges page experience, and, more importantly, they line up with what makes visitors stay or leave.

Largest Contentful Paint (LCP)

What it measures: how long it takes for the biggest thing on the screen to appear. That’s usually your hero image or main heading.

Good score: 2.5 seconds or less. Over 4 seconds is poor.

What it feels like: the moment the page looks “loaded”. A slow LCP is the blank or half-built screen people stare at before giving up.

Common causes of a slow LCP:

• A large hero image that hasn’t been resized or compressed.
• Slow hosting, so the server takes a long time to send the first byte.
• JavaScript or fonts that have to load before the main content can be shown.

Interaction to Next Paint (INP)

What it measures: how quickly the page responds when someone taps, clicks or types. It looks at interactions across the whole visit, not just the first one.

Good score: 200 milliseconds or less. Over 500 milliseconds is poor.

What it feels like: tapping a menu button and nothing happening, so you tap again. Or typing in a form and the letters lagging behind.

Common causes of a slow INP:

• Too much JavaScript running on the page, often from page builders, sliders and plugins.
• Third-party scripts such as chat widgets, tag managers and tracking pixels competing for the phone’s processor.

INP replaced an older measure called First Input Delay (FID) in 2024. If you see FID mentioned in older articles, INP is its stricter successor.

Cumulative Layout Shift (CLS)

What it measures: how much the page jumps around while it loads.

Good score: 0.1 or less. Over 0.25 is poor.

What it feels like: going to tap a link, then an image or banner loads above it and pushes it down, so you tap the wrong thing.

Common causes of a high CLS:

• Images and videos without a set width and height.
• Banners, cookie notices or ads that appear after the page has started rendering.
• Web fonts that swap in and change the size of the text.

Lab scores vs real-world data

PageSpeed Insights shows two kinds of results, and it helps to know the difference.

• Field data (“Discover what your real users are experiencing”) comes from real Chrome users visiting your site over the past 28 days. This is what Google uses. Smaller sites may not have enough visitors to show it.
• Lab data (the Lighthouse score out of 100) is a single test run on a simulated mid-range phone. It’s great for diagnosing problems, but it can vary a little each time you run it.

Google judges each vital at the 75th percentile of visits. In other words, three out of four visits need to hit the “good” mark for the page to pass.

How to improve your scores

Some fixes are quick:

• Resize and compress images, and use a modern format such as WebP.
• Give every image and embed a width and height.
• Remove plugins and third-party scripts you don’t really need.

If your scores are poor across the board, the problem is usually the platform itself: a heavy theme or page builder that loads a lot of JavaScript on every page. That’s when a rebuild on a faster foundation pays off. We build sites with Astro and serve them from Cloudflare, which makes good Core Web Vitals the starting point rather than something you fight for. You can read more in Why is my website slow?

Check your own site

Run our free speed check to see your Lighthouse scores and Core Web Vitals in a few seconds. If you’d like help reading the results, get in touch.