Core Web Vitals Explained: LCP, INP and CLS in Plain English

Core Web Vitals are Google's three speed-and-stability metrics for real users. They are not a vanity score — Google uses them as a ranking signal, and more importantly, they describe the three things that make people close a tab: waiting for content, waiting for reaction, and watching the page jump around.

The first metric is LCP, or Largest Contentful Paint. It measures how long the biggest visible element — usually your hero image or headline — takes to appear. Good is under 2.5 seconds, and it is almost always ruined by one of three things: an unoptimized hero image, a slow server response, or render-blocking CSS and JavaScript. If you only fix one Core Web Vital, fix this one, because it is the one your visitors feel first.

The second is INP, or Interaction to Next Paint. INP replaced FID in 2024 and measures the delay between a tap and the page visually responding. Good is under 200 milliseconds. Long tasks in JavaScript are usually to blame — a heavy script grabbing the main thread while your visitor tries to tap a menu. The fix is less JavaScript, and whatever remains should be broken into smaller chunks that yield to user input.

The third is CLS, or Cumulative Layout Shift. It measures how much content moves around after the page loads — the banner that slides down and pushes your button away just as you click. Good is under 0.1. The causes are boring and entirely fixable: images without width and height attributes, ads and embeds that inject themselves late, and web fonts that swap in with different metrics. Reserve the space, and the page stops moving.

One distinction matters more than any threshold: lab data versus field data. Lighthouse, running in a simulated lab, gives you a score. Google's ranking uses field data — measurements collected from real visitors on real devices, reported in the Chrome UX Report. You can pass a lab test with flying colors and still fail your real users if their phones are slower than your laptop.

To see where you actually stand, start with PageSpeed Insights, which shows both views side by side. If you have traffic, the field data section is your truth. If your site is new and has no field data yet, treat the lab score as a proxy and test on a real mid-range phone.

The practical order of attack rarely changes. First, make the largest above-the-fold element load fast — that is your LCP. Second, cut and defer JavaScript until taps feel instant — that is your INP. Third, give every image, embed, and font slot a fixed size — that is your CLS. Do those three things and you have fixed the metrics that both Google and your visitors care about.

The nice part is that none of this conflicts with design. A fast, stable page is simply a well-built page. The discipline that wins Core Web Vitals is the same discipline that makes a site feel expensive.