Core Web Vitals: Complete Guide

Core Web Vitals are a set of metrics Google uses to measure real-world user experience on a webpage, covering how fast content loads, how quickly the page responds to interaction, and how visually stable it is while loading. Understanding these metrics helps you diagnose why a page might feel slow or clunky to visitors.

The three Core Web Vitals

Largest Contentful Paint (LCP)

LCP measures how long it takes for the largest visible element on a page — often a hero image, heading or large block of text — to render. A good LCP is 2.5 seconds or less. Slow LCP is often caused by large unoptimised images, slow server response times, or render-blocking resources loading before the main content.

Interaction to Next Paint (INP)

INP measures how responsive a page is to user interactions throughout the entire visit, such as clicks, taps and key presses. A good INP is 200 milliseconds or less. INP replaced First Input Delay (FID) as an official Core Web Vital in March 2024, because it captures responsiveness across the whole page lifecycle rather than just the very first interaction.

Cumulative Layout Shift (CLS)

CLS measures visual stability — how much page content unexpectedly shifts as it loads. A good CLS score is 0.1 or less. Common causes include images or ads without reserved dimensions, and web fonts that load in and shift text around.

How Core Web Vitals are measured

Core Web Vitals are assessed using field data — real measurements from actual visitors — rather than a single lab test. Google looks at the 75th percentile of page loads over a rolling period, meaning at least 75% of visits need to meet the "good" threshold for a page to be considered good overall. This is different from lab tools that simulate a single page load under controlled conditions, which can give a useful diagnostic but will not exactly match real-world field data.

Why Core Web Vitals matter

Beyond any potential search ranking influence, Core Web Vitals reflect genuine user experience. Pages that load slowly, respond sluggishly to taps, or jump around as they load tend to frustrate visitors and can increase the chance they leave before completing what they came to do. Improving these metrics is worthwhile for usability and conversion reasons on its own.

How to check your Core Web Vitals

The most reliable source for field data is Google's own tools, but you can also get quick diagnostic feedback with a page speed tool, which draws on Google PageSpeed Insights data where it is available. This gives you both lab-based opportunities for improvement and, where enough real-user data exists, field metrics for your pages.

Common ways to improve each metric

For LCP:

• Optimise and compress large images, especially hero images above the fold
• Improve server response times
• Remove or defer render-blocking scripts and stylesheets

For INP:

• Reduce heavy JavaScript that runs during user interactions
• Break up long-running tasks so the browser can respond sooner
• Avoid excessive third-party scripts that compete for processing time

For CLS:

• Reserve space for images and embeds with explicit width and height
• Avoid inserting new content above existing content after the page has loaded
• Load web fonts in a way that minimises visible text shifts

For a broader look at improving loading times generally, see our guide on how to improve website speed.

Where Core Web Vitals fit into a wider audit

Speed and responsiveness are one part of overall site health. A website performance check looks specifically at this area, while a technical SEO audit covers the broader technical picture including crawlability and indexability alongside speed. ## Field data versus lab data

It helps to understand the difference between the two types of measurement you will encounter. Field data comes from real visitors using real devices and connections, aggregated over a rolling time window, and this is what determines whether a page passes the "good" threshold at the 75th percentile. Lab data comes from a single simulated page load under controlled conditions, which is useful for diagnosing specific issues but will not always match field results exactly, since it cannot capture the full range of real-world devices and network conditions.

Why some pages lack Core Web Vitals data

Pages with low traffic may not have enough real-user visits to generate reliable field data. In these cases, lab-based diagnostics become more important, since they are the only available signal for that specific page. Looking at Core Web Vitals for similar higher-traffic pages on the same site and template can also give a reasonable indication of what a low-traffic page is likely to experience.

Monitoring Core Web Vitals over time

Since Core Web Vitals are based on a rolling window of real user data, scores can shift as your site changes, as your traffic mix changes, or as third-party scripts you rely on are updated by their providers. Periodic re-checks, rather than a single one-off review, are the most reliable way to catch regressions before they affect a meaningful number of visitors.

Next step

Check how your pages are performing with SiteScore's website performance check.