Core Web Vitals: web performance guide

Core Web Vitals are a subset of Google’s Web Vitals that measure three fundamental aspects of user experience: loading speed of the main content, responsiveness to interactions and visual stability of the layout during loading.

These three metrics are measured with real user data (field data) through the Chrome User Experience Report (CrUX), and can also be evaluated in the lab with tools like Lighthouse, PageSpeed Insights or WebPageTest.

LCP measures how long it takes for the largest visible content element in the viewport to render. It can be a hero image, a text block or a video. The optimal threshold is under 2.5 seconds.

The most common causes of a slow LCP are: high server response times, render-blocking CSS and JavaScript, unoptimised images and third-party resource loading. Every millisecond counts, especially on mobile.

INP replaced FID (First Input Delay) in March 2024 as the official interactivity metric. While FID only measured the first interaction, INP evaluates the latency of all interactions during the session and reports the worst one (98th percentile). The optimal threshold is under 200 milliseconds.

A high INP means the page is slow to visually respond to user clicks, taps or key presses. The usual culprits are long JavaScript tasks (long tasks) that block the browser’s main thread.

CLS measures how much visible elements shift unexpectedly during page load. An ad banner pushing content down, an image without dimensions resizing on load or a web font changing text size are typical examples. The optimal threshold is below 0.1.

Layout shifts are one of the most frustrating user experiences. They cause accidental clicks, loss of reading context and an overall perception of low quality.

There are two types of measurement: field data and lab data. Field data reflects the real experience of your users and is what Google uses for ranking. Lab data is useful for diagnosing and debugging specific issues in a controlled environment.

Core Web Vitals are a confirmed Google ranking factor. They are not the most important factor (content relevance remains paramount), but they act as a tiebreaker between pages with similar content. In competitive sectors like ecommerce or digital media, that difference can be significant.

Beyond SEO, performance directly affects conversion. Vodafone reported an 8% increase in sales after improving their LCP by 31%. Every 100ms improvement in loading time can represent a 1% increase in conversion, according to studies by Deloitte and Akamai.

Optimising Core Web Vitals requires a systematic approach. Applying isolated techniques is not enough: you need to identify the real bottlenecks on your site, prioritise them by impact and measure after each change.