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Core Web Vitals: what they are and how to fix them

Simon Tern

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Simon Tern

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Core Web Vitals is Google’s way of measuring how a page feels to use. Is it fast? Does stuff jump around? Does it respond when I tap? Three metrics, three questions.

The three metrics

LCP, largest contentful paint. How long until the biggest element shows up, often an image or a headline. Target: under 2.5 seconds.

INP, interaction to next paint. How fast the page reacts when you tap, type or open a menu. Target: under 200 milliseconds. INP replaced the old metric FID.

CLS, cumulative layout shift. How much the page moves while it loads. You are about to tap a button, an ad loads in, and you tap something else entirely. Target: under 0.1.

Does it matter for rankings?

Yes, but in moderation. It is one signal among many. Good scores do not make a bad page good. But poor scores cost you visitors and conversions, whatever Google thinks. That is why it is worth fixing.

Where do you see your scores?

  • Search Console. The Core Web Vitals report shows real users, split by mobile and desktop.
  • PageSpeed Insights. Paste in a URL. Look at the field data at the top first, lab data after.
  • Chrome DevTools. For those who like to dig.

Field data is what counts. Lab data is a guess under ideal conditions.

Scores are judged at the 75th percentile. That means three out of four visits should pass the threshold. The tools sort pages into good, needs improvement and poor. Start with the pages that are poor and have the most traffic.

Common causes and fixes

Slow LCP

  • Images that are too big. Compress them and use modern formats like WebP or AVIF.
  • No priority on the main image. Preload it, set fetchpriority="high" and do not lazy load it. Lazy loading is for images further down.
  • A slow server, meaning high TTFB (time to first byte). Better hosting, caching and a CDN help. No image optimisation saves a sluggish server.
  • Heavy fonts. Load fewer variants.

Poor INP

  • Too much JavaScript. Remove plugins and scripts you do not need.
  • Third-party scripts such as chat, tracking and widgets. Review them honestly.
  • Heavy menus and filters that run on the main thread.
  • Long JavaScript tasks. Break them up so the browser can answer a tap in between.

High CLS

  • Images without width and height set. Set them.
  • Ads and banners that appear without reserved space. Reserve the space.
  • Cookie banners that push the content down. Let them sit on top.
  • Fonts that change appearance as they load.

The order to work in

  1. Measure mobile first. That is where scores are usually worst.
  2. Fix the pages with the most traffic.
  3. Remove scripts you do not need before you optimise the ones you do.
  4. Measure again after four weeks. Field data updates slowly.

A tip from real life

The usual culprit is rarely the code. It is a 4 MB image someone uploaded to the homepage. Start there.

Need a review? It is part of my technical SEO and of the SEO service. Get in touch.

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