When someone says a site feels slow, they usually mean a vague impression. Google turned that impression into three specific numbers, collectively called Core Web Vitals. Knowing them is the difference between guessing and measuring.
1. LCP — how long the biggest thing takes to arrive
Largest Contentful Paint measures when the largest visible element — usually the hero image or a big headline — has finished rendering. Google's threshold for “good” is 2.5 seconds or less.
The most common cause is always the same: one large, unoptimised image at the top of the page. The second is a font that arrives late and holds the text invisible until it does.
2. INP — how long before the site answers a click
Interaction to Next Paint measures the gap between a user's click and the first visible response. Good is 200 milliseconds or less. It replaced FID in 2024, because FID only ever looked at the first interaction and ignored the rest of the page's life.
The culprit is usually JavaScript: heavy analytics and tracking, drop-in chat widgets, and plugins that do too much work on every click.
3. CLS — how much the page jumps under the reader
Cumulative Layout Shift measures unexpected movement — the familiar annoyance of reaching for a button as the page lurches downward. Good is 0.1 or less.
The fix is simpler than it sounds: give every image and video explicit dimensions so the browser reserves the space before the file arrives. That single change usually removes most of it.
Where to start
Put your URL into PageSpeed Insights. You'll see lab data and, if the site has enough traffic, data from real visitors. Trust the real-visitor data more: the lab number is measured on a simulated network and rarely matches what people actually experience.
One caveat. These three are a ranking signal, not the whole of it. A fast site with nothing to say still won't rank.