Core Web Vitals Explained: LCP, INP, and CLS

Core Web Vitals are a set of metrics Google uses to quantify real-world page experience. They’re part of how page experience factors into search, but more importantly, they correlate closely with whether people actually stick around on a page. Here’s what each one measures.

Largest Contentful Paint (LCP)

LCP measures how long it takes for the largest visible element on a page — usually a hero image, a heading, or a large block of text — to render. It’s a proxy for perceived load speed: how long a visitor waits before the page feels “there.”

A good LCP is under 2.5 seconds. Common causes of slow LCP include unoptimized images, render-blocking JavaScript and CSS, slow server response times, and fonts that block text rendering until they load.

Interaction to Next Paint (INP)

INP measures how responsive a page feels when a user actually interacts with it — clicking a button, opening a menu, typing into a field. It replaced First Input Delay as the interactivity metric because it captures responsiveness across the entire page lifecycle, not just the first interaction.

A good INP is under 200 milliseconds. Heavy JavaScript execution, large third-party scripts, and long tasks that block the main thread are the usual culprits when INP is poor.

Cumulative Layout Shift (CLS)

CLS measures visual stability — how much content unexpectedly shifts around while a page loads. The classic example is clicking a button just as an ad loads above it and pushes it down, so the click lands on the wrong thing.

A good CLS score is under 0.1. It’s usually caused by images or embeds without explicit width/height attributes, web fonts that swap in and change text size, or content injected above existing content after the initial render.

Why these specific three

Google didn’t pick these metrics arbitrarily — they map to the three things that most affect whether a page experience feels good: how fast it loads, how responsive it is, and how stable it is while you’re using it. A page can be fast to first paint and still feel broken if it’s laggy or jumpy, which is why all three are tracked separately rather than collapsed into one score.

Where most sites actually lose points

In practice, the most common failure points are:

• Hero images served at full resolution instead of responsively sized
• Third-party scripts (chat widgets, ad tech, analytics) loaded synchronously and blocking the main thread
• Web fonts causing a flash of invisible or shifting text
• Ads or dynamically injected content with no reserved space

Fixing Core Web Vitals is rarely about one big change — it’s usually a handful of specific, measurable fixes that compound.