Core Web Vitals: the technical measures Google uses to assess your website

You've probably seen "Core Web Vitals" in an SEO article or a Google report. And if you're like most of my clients, you nodded politely and moved on. Fair enough: it's technical. But it also DIRECTLY affects your Google rankings. So let's talk about it.

What are Core Web Vitals?

Basically, they're three metrics Google uses to measure visitors' experience on your website. Not content or backlinks: thetechnical experience. Does the site load quickly? Does it respond when you click? Does the page jump around while loading?

Google decided these three things were important enough to become SEO ranking factors. Translation: if your site is slow and unstable, you lose ground in search results. Simple as that.

LCP — Largest Contentful Paint (loading speed)

LCP measures how long the largest visible element takes to appear. Usually that's your main image or hero heading. It's the moment a visitor thinks, "OK, the page has loaded."

Good : under 2.5 seconds. Needs improvement : 2.5 to 4 seconds. Poor : over 4 seconds.

The number-one cause of poor LCP? Unoptimised images. A 3 MB JPEG photo that could be a 150 KB WebP is incredibly common. I've seen sites shave two seconds off loading time just by converting images.

INP — Interaction to Next Paint (responsiveness)

INP replaced FID (First Input Delay) in March 2024. It measures the delay between a visitor clicking or typing and the browser responding visually.

Good : under 200 milliseconds. Needs improvement : 200 to 500 ms. Poor : over 500 ms.

If a visitor clicks a button and nothing happens for half a second, they'll think the site is broken. The culprit? Often heavy JavaScript blocking the browser. Live chat, tracking, pop-up scripts… each adds a few dozen milliseconds. It adds up.

CLS — Cumulative Layout Shift (visual stability)

You know that annoying moment: you're reading an article and BAM, an ad loads and all the text shifts five centimetres. Or you're about to click a link when a banner appears and you click somewhere else. That's CLS.

Good : under 0.1. Needs improvement : 0.1 to 0.25. Poor : over 0.25.

Common causes: images without dimensions (width/height), late-loading web fonts and dynamically inserted ads. The fix is often very simple: set image dimensions and preload your fonts.

How to test your Core Web Vitals

The simplest way: go to PageSpeed Insights, paste your URL and wait 30 seconds. You'll get a score out of 100 and a breakdown of every metric with recommendations.

Other useful tools:

Google Search Console — open "Experience", then "Core Web Vitals". This shows actual problems your visitors encounter, not a simulation. It's the most reliable data.

Lighthouse (in Chrome DevTools) — right-click your page > Inspect > Lighthouse tab > Analyse. A detailed report with suggested fixes.

GTmetrix — provides a score and a waterfall showing when each element loads. Very useful for identifying what's slowing everything down.

Practical fixes (no developer skills required)

To improve LCP (speed)

Compress your images. Use WebP instead of JPEG or PNG. Free tools such as Squoosh or TinyPNG do this in a couple of clicks. Aim to keep every image below 200 KB.

Enable browser caching. When visitors return, their browser reuses files it has already downloaded instead of fetching everything again. On WordPress, a plugin such as WP Rocket does this automatically.

Use a CDN. A CDN (Content Delivery Network) stores copies of your site on servers around the world. Visitors load it from the nearest server. Cloudflare's free plan is already very good: it's what I use for semzen.fr.

To improve INP (responsiveness)

Remove unnecessary scripts. Every WordPress plugin, tracking script and chat widget adds JavaScript. Have a clear-out. Do you really need four different tracking scripts? Probably not.

Defer non-critical JavaScript. The attributes defer and async on your <script> tags let the browser display the page without waiting for every script to load. It's technical, but your developer will know how.

To improve CLS (stability)

Set your image dimensions. Always add width and height to your <img>tags. The browser reserves space before the image loads, preventing layout shifts.

Preload your fonts. Choose <link rel="preload"> for Google Fonts prevents flashes of invisible text (FOIT) or fonts changing mid-load.

Avoid dynamic elements above the fold. Ads, cookie banners and pop-ups inserted at the top of the page all move content around. Place them at the bottom or use a fixed overlay.

Is it really worth it?

Honestly, yes. A Vodafone study showed that a 31% improvement in LCP led to 8% more sales. Amazon calculated that an extra 100 ms of loading time cost 1% in revenue. You're not Amazon, of course, but the principle is the same: a fast website converts better.

On the SEO side, Google has confirmed that Core Web Vitals are a ranking factor. Not THE main factor — content and backlinks still come first — but a tie-breaker for similar content. If you and a competitor have similar content and your site is faster, you move ahead.

Core Web Vitals aren't just a geeky detail. They're a practical way to improve both SEO AND conversions. Test your site now on PageSpeed Insights, identify the problems and start with images: that's the quick win with the best results.

And if your score is terrible and you don't know where to start, let's talk. A quick audit often identifies three or four high-impact actions.

Pretty good, right?

Source: web.dev — Core Web Vitals, Google 2025

Translated from the original French article. Publication dates, examples and figures refer to that original version.