Core Web Vitals, Explained Without the Jargon

Core Web Vitals are Google's attempt to measure something that used to be purely subjective — "does this page feel fast?" — using three specific, measurable moments. Each one maps to a real frustration you've almost certainly experienced yourself.

LCP: how long until the main thing shows up

Largest Contentful Paint measures how long it takes for the biggest visible element — usually a hero image, a headline, or a large block of text — to fully render. This is the "is this page ever going to load" moment. Google's threshold for "good" is 2.5 seconds or faster.

The most common cause of a slow LCP is an unoptimized hero image: a multi-megabyte photo served at full resolution when the page only displays it at a fraction of that size. Second most common: a render-blocking script or stylesheet that delays everything below it, including the main content.

INP: how responsive the page feels once it's loaded

Interaction to Next Paint measures the delay between a user action — a click, a tap, a keypress — and the page visibly responding. This is the "I clicked and nothing happened, so I clicked again" moment, which often causes double-submissions or duplicate actions. Google's threshold for "good" is 200 milliseconds or faster.

INP problems usually trace back to heavy JavaScript running on the main thread at the exact moment someone tries to interact — a large script parsing, a expensive re-render, an analytics script blocking momentarily.

CLS: whether the page holds still

Cumulative Layout Shift measures how much visible content unexpectedly moves around as a page loads. This is the "I was about to tap a button and an ad loaded above it, so I tapped the wrong thing" moment — genuinely one of the more infuriating web experiences, and common enough that Google made it a dedicated metric. Good CLS is a score of 0.1 or lower.

CLS is almost always caused by content loading in without reserved space: an image with no defined dimensions, a font swap that changes text size, an ad or embed injecting itself into the layout after the page already rendered.

Field data vs. lab data

A score from a single Lighthouse run is a lab measurement — one simulated visit, one set of conditions. Real Core Web Vitals scoring (the kind that affects search) comes from field data: aggregated measurements from actual visitors on actual devices and connections. A page can score well in a lab test and still perform poorly in the field if real visitors are disproportionately on older phones or slower connections than the test simulated — which is why both numbers are worth checking, not just one.