What are Core Web Vitals?

Core Web Vitals are a set of performance metrics that Google uses to evaluate the real-world experience of visiting a web page. Introduced in 2021 as part of Google's page experience update, they directly influence where your site ranks in search results.

There are currently three metrics in the Core Web Vitals set. Each one measures a different aspect of how a page feels to use — loading, interactivity, and visual stability.

Loading
Largest Contentful Paint
LCP
< 2.5 s Good
2.5 – 4 s Needs improvement
> 4 s Poor
Interactivity
Interaction to Next Paint
INP
< 200 ms Good
200 – 500 ms Needs improvement
> 500 ms Poor
Visual stability
Cumulative Layout Shift
CLS
< 0.1 Good
0.1 – 0.25 Needs improvement
> 0.25 Poor

LCP — Largest Contentful Paint

LCP measures how quickly the largest visible element on the page loads. This is usually a hero image, a heading, or a large block of text — whatever takes up the most space above the fold.

A good LCP is under 2.5 seconds. If your hero image takes four seconds to appear, visitors see a blank or half-loaded page, and many will leave before anything renders properly.

Common causes of poor LCP:

  • Large, unoptimised images (the most frequent culprit)
  • Slow server response times
  • Render-blocking scripts that delay the page from drawing
  • No lazy loading strategy — images loading all at once

Quick fixes: convert images to WebP or AVIF format, compress them, use a CDN, and preload the hero image so the browser prioritises it.

INP — Interaction to Next Paint

INP replaced First Input Delay (FID) in March 2024. It measures the delay between a user doing something — clicking a button, tapping a link, selecting from a dropdown — and the page visually responding. It captures all interactions throughout the page visit, not just the first one.

Under 200 ms feels instant. Anything over 500 ms feels sluggish and broken.

Common causes of poor INP:

  • Heavy JavaScript executing on the main thread
  • Poorly written event handlers that do too much work
  • Third-party scripts (chat widgets, ad trackers) blocking the thread
  • Large DOM trees that slow down rendering

Quick fixes: defer or remove non-essential scripts, break long tasks into smaller chunks, and avoid loading dozens of plugins or widgets on every page.

CLS — Cumulative Layout Shift

CLS measures visual stability — how much the page layout shifts unexpectedly as it loads. You've experienced a high CLS site when you go to tap a button and it jumps just as your finger lands, because an image above it loaded late and pushed everything down.

A CLS score under 0.1 is considered good. This is measured as a unitless score, not seconds — it represents the proportion of the screen that shifts, multiplied by the distance elements move.

Common causes of poor CLS:

  • Images without explicit width and height attributes
  • Adverts or embeds inserted without reserved space
  • Custom fonts causing text to reflow as they load (FOUT)
  • Dynamic content injected above existing content

Quick fixes: always set width and height on images, reserve space for ads and iframes, use font-display: swap with preloaded fonts, and avoid injecting content at the top of the page after load.

How to measure your Core Web Vitals

The easiest place to start is Google PageSpeed Insights (pagespeed.web.dev). Enter your URL and it returns field data (real user data from Chrome, where available) alongside lab data from a simulated test. It also gives you a prioritised list of what to fix first.

Other useful tools:

  • Google Search Console — the Core Web Vitals report shows real user data aggregated across all your pages.
  • GTmetrix — detailed waterfall charts showing exactly what loads when.
  • WebPageTest — in-depth diagnostics for developers, with visual filmstrip playback.
  • Chrome DevTools — the Performance and Lighthouse tabs give you instant local feedback.

Field data vs lab data: Lab data is measured in a controlled environment (a simulated device, fixed connection). Field data comes from real Chrome users visiting your site. Google ranks based on field data — so if your PageSpeed score looks fine but real users are on slow mobile connections, you may still have a ranking issue.

Do Core Web Vitals directly affect rankings?

Yes — but they are one signal among many. Google has confirmed that Core Web Vitals are a ranking factor, with pages that pass all three thresholds eligible for a small ranking boost over comparable pages that don't. They will never compensate for poor content or a spammy backlink profile, but for sites competing on similar content quality, they can make a meaningful difference.

More importantly, better Core Web Vitals mean a better user experience — and that means lower bounce rates, more time on site, and higher conversion rates. The SEO benefit is almost secondary.

What to do next

Run your site through PageSpeed Insights today. Note your LCP, INP, and CLS scores. If any are in the "needs improvement" or "poor" range, look at the specific suggestions Google provides — they're usually actionable and specific to your page.

If you want a full technical audit and performance optimisation as part of a new build, take a look at what's included in my web design service. Core Web Vitals optimisation is built into every project.

You might also find it useful to read what technical SEO actually covers — Core Web Vitals are just one piece of that broader picture.

Ready to talk through your site's performance? Get in touch.