Core Web Vitals, explained.

Core Web Vitals are three measurements that describe how a web page feels to a real person: whether it loads quickly, responds quickly and stays still while they use it. Google’s ranking systems use them, and they are a fair summary of what makes a site pleasant or frustrating.

The three measurements

• Largest Contentful Paint (LCP) is how long the largest thing on screen, usually the main image or headline, takes to appear. Good is 2.5 seconds or less.
• Interaction to Next Paint (INP) is how long the page takes to respond visibly after a tap, click or key press. Good is 200 milliseconds or less.
• Cumulative Layout Shift (CLS) is how much the page jumps around as it loads. Good is a score of 0.1 or less.

A page passes when at least 75 per cent of visits meet the “good” mark for each one. Phones and computers are counted separately.

Lab tests and real visitors

Tools such as PageSpeed Insights show two kinds of result. Lab data is a single test run on a simulated device, which is useful for finding problems. Field data comes from real Chrome users over the previous 28 days, and it is the one that counts. A page can score well in the lab and poorly in the field if real visitors use slower phones or connections than the test assumes.

What usually causes a poor score

• Slow LCP: a large image that has not been compressed or sized properly, slow hosting, or fonts and scripts that hold up the first view.
• Slow INP: too much JavaScript, often from chat widgets, tracking tags and page builders.
• High CLS: images and advertisements with no space reserved for them, or banners that push content down after it has appeared.

Where to look

1. Run the page through PageSpeed Insights, and read the field data at the top first.
2. Check the Core Web Vitals report in Google Search Console to see which groups of pages fail.
3. Fix the worst measurement on the most visited pages before anything else.
4. Wait. Field data covers 28 days, so an improvement takes about a month to show in full.

This website is built the way we build for clients: fonts and icons are served from the site itself, images have fixed dimensions, and there is very little JavaScript. See Websites and Optimisation.