What Core Web Vitals Are
Core Web Vitals are Google's standardised set of user experience metrics - specific, measurable signals about how a page feels to use. They were officially incorporated into Google's ranking algorithm in June 2021 via the Page Experience update.
There are three signals, each measuring a different dimension of the experience:
Largest Contentful Paint (LCP) measures loading performance. Specifically, how long it takes for the largest visible element (usually the hero image or headline) to render. Target: under 2.5 seconds.
Cumulative Layout Shift (CLS) measures visual stability. How much does the page jump around as elements load? A high CLS score is that jarring experience of trying to click a button that moves just before you tap it. Target: under 0.1.
Interaction to Next Paint (INP) replaced First Input Delay in March 2024. It measures responsiveness - how quickly the page responds to user interactions throughout the session. Target: under 200 milliseconds.
Why They Matter for Rankings
Google confirmed that Core Web Vitals are used as a tiebreaker in ranking decisions when two pages are otherwise equally relevant. In practice, this means that for competitive search terms, a measurably better user experience can be the deciding factor between position 5 and position 1.
Beyond direct ranking impact, there's an indirect mechanism: better page experience reduces bounce rate and increases dwell time, both of which correlate with higher rankings.
How to Measure Them
Google Search Console provides field data (real user measurements) across your site, highlighting pages that need attention. This is the data Google uses for ranking decisions.
PageSpeed Insights provides both field data and lab data (simulated tests) for individual URLs, along with specific improvement recommendations.
Chrome DevTools and Lighthouse provide lab measurements for detailed diagnostic work during development.
Common LCP Failures
The most common LCP problem is an unoptimised hero image. Solutions: serve images in WebP or AVIF format, use a CDN, preload the LCP image in the '<head>' with '<link rel="preload">', and eliminate any render-blocking CSS or JavaScript that delays the image from loading.
Hosting matters too. A server that responds in 200ms versus one that responds in 800ms creates an LCP difference that no amount of image optimisation can fully compensate for.
Common CLS Failures
Layout shift almost always comes from one of four causes: images without declared dimensions (the browser doesn't know how much space to reserve), dynamically injected content above existing content, web fonts causing text reflow, and late-loading third-party embeds.
Fix dimensions: always set 'width' and 'height' attributes on '<img>' elements. Use 'aspect-ratio' in CSS for responsive images. Use 'font-display: swap' for web fonts and reserve space for them.
Common INP Failures
High INP is usually caused by long JavaScript tasks blocking the main thread. The browser can't respond to input while it's executing JavaScript. Solutions: break up long tasks using 'scheduler.postTask()' or 'setTimeout', defer non-critical JavaScript, and audit third-party scripts (analytics, chat widgets, and marketing tags are frequent offenders).
