Skip to content

Core Web Vitals Checker: what LCP, CLS, and INP mean

A core web vitals checker reports on Google's three experience metrics — LCP, CLS, and INP — which together describe loading, stability, and responsiveness. True vitals require rendering the page in a lab browser or collecting real-user data; a single fetch cannot produce them. This page explains each metric honestly and shows which instant signals our check does cover.

Run the instant check

Instant connection signals, not lab data. This check reports response time, page weight, viewport, and HTTPS from one fetch — it does not render the page, so it cannot measure LCP, CLS, or INP.

Enter a URL to begin

Instant signals only — nothing is stored.

What the core web vitals are

Largest Contentful Paint (LCP) measures loading: how quickly the biggest visible element appears. Google's good threshold is 2.5 seconds or less.

Cumulative Layout Shift (CLS) measures visual stability: how much the layout jumps while loading. A score of 0.1 or less counts as good.

Interaction to Next Paint (INP) measures responsiveness: how fast the page reacts to taps and clicks. Under 200 milliseconds is the good threshold.

All three need a browser — either a lab run under controlled conditions or real-user measurements from actual visits. Numbers produced without rendering are estimates at best.

What the instant check covers

Covered by this check

  • Server response time, which feeds into LCP
  • Page weight, a driver of slow loading
  • Viewport meta, needed for stable mobile layout
  • HTTPS baseline for a trustworthy experience

Not covered — needs a lab test

  • Measured LCP, CLS, or INP values
  • A pass or fail vitals verdict
  • Field data from real users
  • Device- and network-specific lab runs

How to improve each vital

  • Faster LCP

    Compress hero images, preload key assets, and cut server response time so the largest element arrives early.

  • Lower CLS

    Reserve space for images and embeds with width and height attributes so content never jumps.

  • Better INP

    Reduce blocking scripts and defer non-critical work so taps get an immediate response.

  • Lighter pages overall

    Every kilobyte removed helps all three vitals at once. Audit scripts and media ruthlessly.

  • Verify in a lab

    Confirm improvements with PageSpeed Insights or Lighthouse, then watch real-visitor behaviour in your analytics.

How Traffsite connects experience to visits

Vitals work matters because visitors leave slow, jumpy pages. Traffsite measures what happens on your own pages — views and trends per page — so experience improvements show up as audience retained.

Start a free trial to pair lab measurements with real visit data from your own site.

Common questions

What do LCP, CLS, and INP stand for?

Largest Contentful Paint (loading speed), Cumulative Layout Shift (visual stability), and Interaction to Next Paint (responsiveness). Together they summarise page experience.

Can an instant check measure core web vitals?

No. Vitals require rendering the page in a browser or collecting real-user data. This check reports the fetch-level signals that feed into them — response time, weight, viewport, HTTPS — and labels them as such.

What are good thresholds for the vitals?

LCP at or under 2.5 seconds, CLS at or under 0.1, and INP under 200 milliseconds, measured in lab or field tools.

Where do I get real vitals numbers?

Run the URL through PageSpeed Insights or Lighthouse for lab data, and use real-user monitoring for field data. Then track resulting visits in Traffsite.

Related tools

See all tools

Track your site visitors with Traffsite Web Analytics

Start free