Core Web Vitals are three measurements Google uses to describe how a page feels to real visitors: how quickly the main content appears, how quickly the page responds when someone interacts with it, and how stable the layout is while it loads. They're worth understanding because each one maps to a frustration your visitors actually experience.
Largest Contentful Paint (LCP)
LCP measures how long it takes for the largest visible element — usually the hero image or main headline — to appear. Google considers 2.5 seconds or less to be good.
- Common causes of a slow LCP: very large hero images, images in old formats, slow hosting, and too many fonts or scripts loading before the content.
- Common fixes: correctly sized images in modern formats such as WebP or AVIF, prioritising the hero image, and trimming render-blocking resources.
Interaction to Next Paint (INP)
INP measures how quickly a page visibly responds after someone taps, clicks or types. It replaced First Input Delay as a Core Web Vital in March 2024. 200 milliseconds or less is considered good.
- Common causes: heavy JavaScript, large third-party widgets, and long tasks that block the main thread.
- Common fixes: shipping less JavaScript, deferring non-essential scripts, and breaking up expensive work.
Cumulative Layout Shift (CLS)
CLS measures how much the page jumps around while loading — the moment you go to tap a button and it moves. A score of 0.1 or less is considered good.
- Common causes: images and embeds without dimensions, late-loading banners, and web fonts that swap in with very different sizes.
- Common fixes: setting width and height on media, reserving space for dynamic content, and choosing fonts and fallbacks carefully.
Lab data vs field data
Tools like Lighthouse run a test in a controlled environment — useful for diagnosing problems, but not the same as what real visitors experience. Field data, such as the Chrome UX Report shown in PageSpeed Insights, comes from real users and is what Google uses for its assessment. A new or low-traffic site may not have enough field data yet; in that case, lab tests are the best available guide.
Where to start
- Run your homepage through PageSpeed Insights and note which of the three metrics needs attention.
- Check your largest images first — they're the most common and easiest win.
- Remove plugins, widgets and scripts you no longer need.
- Re-test after each change so you know what actually helped.
Performance won't replace good content or a clear offer, but a slow, jumpy site undermines both. Fixing the basics gives everything else a fair chance.