Three numbers decide whether Google thinks your site is fast. You do not need to know how to fix them to know what they mean, and knowing what they mean is enough to ask the right question of whoever built the site.
The three numbers
- Largest Contentful Paint: how long until the biggest thing on screen has appeared. Aim for under 2.5 seconds.
- Interaction to Next Paint: how long the page takes to respond when someone taps or clicks. Under 200 milliseconds.
- Cumulative Layout Shift: how much things jump around while the page loads. Under 0.1.
Lab data and field data
A Lighthouse score is lab data: one simulated visit, on one simulated device, from one place. Google ranks on field data: what real visitors on real phones actually experienced over the last 28 days. The two often disagree, and when they do, the field wins.
A site can score 95 in Lighthouse and fail Core Web Vitals in the field. The lab test ran on fast wifi; your clients are on a train.
Worth knowing
Where to look
Search Console has a Core Web Vitals report that shows the field numbers by page group. If it says “not enough data”, your site does not get enough traffic for Google to measure it, which is its own finding.
If it shows failures, the fix is usually one of three things: images that are too large, a hero that loads late, or a page builder doing too much before it lets the browser draw. All three are decisions made at build time, not settings you can toggle later.
