Key information
Core Web Vitals are a set of specific, measurable metrics Google uses to assess real-world page experience, and they factor into how your pages rank in search results. They measure three things: how quickly a page loads its main content, how quickly it responds to user interaction, and how visually stable it is while loading. Poor Core Web Vitals scores are one of the most common, and most fixable, reasons a technically sound website underperforms in search.
The three Core Web Vitals metrics
1. Largest Contentful Paint (LCP): Measures how long it takes for the largest visible element — usually a hero image or heading — to render. Google considers under 2.5 seconds good. Slow LCP is most often caused by unoptimised images, render-blocking CSS/JavaScript, or slow server response times.
2. Interaction to Next Paint (INP): Measures how quickly a page responds when a user clicks, taps or types. Under 200 milliseconds is good. Poor INP usually comes from heavy JavaScript execution blocking the browser's main thread.
3. Cumulative Layout Shift (CLS): Measures unexpected visual movement as a page loads — images popping in without reserved space, or ads and banners shifting content around. A score under 0.1 is good. This is typically fixed by setting explicit width and height attributes on images and embeds.
Why Core Web Vitals matter beyond rankings
Even setting SEO aside, Core Web Vitals correlate directly with conversion rate and bounce rate. A visitor who has to wait for a slow-loading page, or who taps a button that doesn't respond for half a second, is measurably more likely to leave. Google made these metrics part of its ranking systems precisely because they reflect a real, felt difference in user experience — not an arbitrary technical hoop to jump through.
How to check and improve your scores
Google's PageSpeed Insights and Search Console's Core Web Vitals report both show real-world, field data collected from actual visitors, which is more reliable than lab-only testing tools. Common fixes include compressing and correctly sizing images (ideally serving modern formats like WebP), deferring non-critical JavaScript, using explicit width/height attributes to prevent layout shift, and enabling browser caching and compression at the server level via cache-control headers and gzip.
These are exactly the fixes we run as standard on every website build and SEO engagement — Core Web Vitals work isn't a separate add-on, it's part of doing technical SEO properly in the first place.
What Changed With INP Replacing FID
In March 2024, Google replaced First Input Delay (FID) with Interaction to Next Paint (INP) as the official responsiveness metric. FID only measured the delay before the browser began processing the very first interaction on a page; INP measures responsiveness across every interaction throughout the entire page visit, giving a far more complete picture of how a page actually feels to use. This change caught many sites off guard — pages that scored well on FID sometimes score poorly on INP if later interactions (opening a menu, submitting a form) are sluggish, even if the very first click felt instant.
Field Data vs Lab Data
It’s worth understanding the difference between the two types of Core Web Vitals data Google reports. Field data (also called Chrome User Experience Report, or CrUX data) comes from real visitors using Chrome, and this is what actually affects your search rankings. Lab data comes from simulated tests run in tools like Lighthouse under controlled, consistent conditions. Lab data is useful for debugging specific issues in a repeatable environment, but a page that scores perfectly in a Lighthouse lab test can still perform poorly in the field if real users are on slower connections, older devices, or if third-party scripts behave differently under real-world network conditions.