What are Core Web Vitals?

The three metrics

• LCP — Largest Contentful Paint. Time until the largest visible element, usually a hero image or headline, has rendered. Good: 2.5 seconds or less.
• INP — Interaction to Next Paint. How long the page takes to visibly respond to clicks, taps and key presses, taken from the slowest interactions of a visit. Good: 200 milliseconds or less. INP replaced First Input Delay in 2024.
• CLS — Cumulative Layout Shift. How much content jumps around while the page loads, as a unitless score. Good: 0.1 or less.

Two supporting metrics help diagnose them: TTFB (time to first byte, server speed) and FCP (first contentful paint, when anything appears).

Field data and lab data

There are two ways to measure:

• Lab data — a test tool loads the page once on a simulated device. Repeatable, good for debugging, but one device and one network.
• Field data (real user monitoring) — the numbers from real visitors on their own phones and connections. This is what Google uses, and it is judged at the 75th percentile: a page passes if three out of four loads are good.

Lab and field often disagree. A page that scores well on a fast laptop can fail on mid-range phones over mobile data.

Why speed is a business number

Web Vitals are part of how Google evaluates page experience, but the bigger effect is on visitors. Slow loading raises bounce rate on landing pages; slow responses make buttons feel broken; layout shifts cause mis-taps on "buy" and "cancel". The useful question is not "what is my score?" but "do slow pages convert worse?" — which needs speed and conversion in the same tool.

How TapCub measures Web Vitals

The TapCub script collects LCP, INP, CLS, FCP and TTFB from real visits, together with a split of the load into time to first byte, download and front-end rendering. They are measured on the first full page load in a tab and sent when the visitor leaves; there is no separate performance script to install, and the collection can be switched off with one attribute.

The performance report shows a 0–100 score built from LCP, INP and CLS, the share of fast loads (LCP ≤ 2.5 s), the load-time distribution and the slowest pages by 75th-percentile LCP. Because the numbers sit in the same data as bounce rate and conversions, you can filter performance by page, device or country and compare it with how those visits behaved.

Common mistakes

• Optimising for a lab score. Field data on real devices is what visitors experience.
• Reading averages. Web Vitals are judged at the 75th percentile; an average hides the slow quarter.
• Testing only the home page. Product, article and checkout pages usually carry more traffic and different assets.
• Ignoring INP. Heavy JavaScript can make a fast-loading page feel slow on every click.

Related guides: Real User Monitoring. See also LCP, CLS and INP.

See also: Mobile first.

See also

• How do I check Core Web Vitals for real visitors?