What are Core Web Vitals and why Google cares about them

Have you heard about Core Web Vitals yet? Google has spent the last couple of years telling us that user experience is going to matter more for rankings. In June 2021, that becomes official with the Page Experience update, and at the heart of it are three metrics called Core Web Vitals.

If you run a website and care about search traffic, you need to understand what these are.

The three metrics

Largest Contentful Paint (LCP) measures how long it takes for the biggest visible element on the page to load, usually a hero image or a large block of text. Google wants this under 2.5 seconds. It's essentially asking: how quickly does the page look ready?

First Input Delay (FID) measures the time between a user's first interaction, clicking a button, tapping a link, and the browser's response. Google wants this under 100 milliseconds. If there's a noticeable lag between clicking and something happening, that's a poor FID.

Cumulative Layout Shift (CLS) measures visual stability. Have you ever been reading an article and the text suddenly jumps down because an ad loaded above it? Or tried to tap a button on your phone just as the page shifted and you hit something else? That's layout shift, and Google wants a CLS score below 0.1.

Why Google cares

Google's business depends on people trusting its search results. If Google consistently sends you to slow, janky, frustrating websites, you'll eventually try a different search engine. By making user experience a ranking factor, Google is incentivising the entire web to get faster and less annoying. It's good for users, which is good for Google.

That said, let's keep this in perspective. Content relevance is still the primary ranking factor by a massive margin. A slow page with exactly the answer someone is searching for will still outrank a fast page with mediocre content. Core Web Vitals are a tiebreaker, not the whole game.

How to check your scores

Google Search Console now has a Core Web Vitals report that shows you how your pages are performing based on real user data. PageSpeed Insights (pagespeed.web.dev) gives you per-page scores with specific recommendations. Chrome DevTools' Lighthouse audit includes all three metrics.

The important thing is to look at field data, real user measurements, not just lab data from a single test. Your experience on a fast MacBook with fibre broadband is not the same as a visitor on a three-year-old Android phone on 4G.

Common problems and fixes

Poor LCP is usually caused by large unoptimised images, slow server response times, or render-blocking JavaScript and CSS. The fixes: compress and properly size your images (WebP format helps significantly), improve your hosting, and defer non-critical scripts.

Poor FID is almost always caused by heavy JavaScript execution. Third-party scripts are the usual culprits, analytics, chat widgets, ad scripts, social media embeds. Each one adds JavaScript that the browser has to parse and execute before it can respond to user input. Audit your third-party scripts ruthlessly. If you have a chat widget that 2% of visitors use but it adds 200ms of processing time for everyone, the maths doesn't work. Worth checking yours, right?

Poor CLS is caused by elements that load without reserved space, images without width and height attributes, ads that push content down, web fonts that cause text to reflow. The fix is to always specify dimensions for images and video, reserve space for dynamic content, and use font-display: swap with properly configured font loading.

What to do now

Run your key pages through PageSpeed Insights. Look at the Core Web Vitals section. If you're in the green on all three, well done, you're ahead of most sites. If not, the recommendations in the report will tell you exactly what to address.

If you need help improving your Core Web Vitals scores or want to understand what the results mean for your specific site, drop me a message at [email protected].