Core Web Vitals explained

What the three measure

Largest Contentful Paint is how long until the main thing on the page appears. This is the one that most often fails and most often matters.

Interaction to Next Paint is how quickly the page responds when someone taps or clicks. Sites with heavy JavaScript fail this; mostly static sites essentially never do.

Cumulative Layout Shift is how much the page jumps around while loading. Almost always caused by images without dimensions, or by content injected after render.

Field data, not lab scores

A Lighthouse score is a simulation on one simulated device. Google uses field data from real Chrome users, which can differ substantially, especially on sites with a mostly mobile audience on slower connections.

Chase the field data in Search Console. Optimising a lab score you can control while the field data stays red is a common and expensive way to be busy.

Test the templates that carry traffic

Almost every Core Web Vitals report we are shown tests the homepage. On most sites the homepage is not where organic visitors land. Test the category, product or service templates that actually receive the traffic.

The usual causes, in order

For LCP: an unoptimised hero image, a lazy-loaded above-the-fold image, or render-blocking scripts and fonts.

For CLS: images and embeds without width and height, and web fonts swapping without matched fallback metrics.

For INP: too much JavaScript, usually from third-party tags nobody audits.