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Core Web Vitals test with Lighthouse

Run Google Lighthouse on your page as a mid-range phone on a slow 4G connection and get lab measurements of the Core Web Vitals: Largest Contentful Paint, Cumulative Layout Shift and Total Blocking Time, the lab stand-in for Interaction to Next Paint. The report adds First Contentful Paint, Speed Index, page weight, a filmstrip of the page as it loads and the improvements with the largest estimated savings.

Runs only this test. Want everything? Run all 18 tests from the home page.

What this test checks

Lighthouse performance score

Lighthouse's own 0–100 mobile performance score, which is also this test's score.

Largest Contentful Paint (LCP)

Time until the largest image or text block on the first screen appears.

Total Blocking Time (TBT)

How long JavaScript keeps the page from responding to taps and clicks, the lab stand-in for Interaction to Next Paint (INP).

Cumulative Layout Shift (CLS)

How much visible content jumps around while the page loads.

First Contentful Paint and Speed Index

When the first text or image appears, and how quickly the visible part of the page fills in.

Page weight and requests

Total bytes downloaded and the number of network requests the page makes.

Render-blocking scripts and image formats

Scripts in the page head that load without defer or async, and whether images use WebP or AVIF.

Accessibility, Best Practices and SEO scores

Lighthouse's other category scores from the same run, with any failed SEO audits listed.

Sample result

A real result for devteam.co.kr, tested 18 hours ago.

Core Web Vitals (Lighthouse)

Mobile performance 99/100, LCP 1.84 s, CLS 0. Measured with Lighthouse from our test server in the US (Virginia).

99/100A+
Mobile performance score (Lighthouse) (Good)
The page loads quickly on mobile.
99 / 100
Target: 90 or higher
Largest Contentful Paint (LCP) (Good)
Time until the largest image or text block on the first screen appears. It meets the target.
1.84 s
Target: 2.5s or less
Total Blocking Time (TBT, responsiveness) (Good)
Time the page is frozen by JavaScript and cannot respond to input (the lab stand-in for INP). It meets the target.
0 ms
Target: 200ms or less
Cumulative Layout Shift (CLS) (Good)
The layout barely shifts while the page loads.
0
Target: 0.1 or less
First Contentful Paint (FCP) (Good)
Time until the first text or image appears on screen. It meets the target.
1.7 s
Target: 1.8s or less
Total page weight (Good)
Page weight is reasonable.
178KB · 14 requests
Target: 2MB or less
1.84 sLargest Contentful Paint
0Cumulative Layout Shift
0 msTotal Blocking Time
1.7 sFirst Contentful Paint
2.27 sSpeed Index

How the page appeared while loading (simulated mobile)

Page at 375 ms
375 ms
Page at 750 ms
750 ms
Page at 1.13 s
1.13 s
Page at 1.5 s
1.5 s
Page at 1.88 s
1.88 s
Page at 2.25 s
2.25 s
Page at 2.63 s
2.63 s
Page at 3 s
3 s
99Lighthouse Performance
100Lighthouse Accessibility
100Lighthouse Best practices
100Lighthouse SEO

Biggest opportunities

OpportunityEstimated savings
Speed up the main document (server time, redirects, compression)
Est savings of 750 ms
750 ms
Reduce server response time
Root document took 850 ms
750 ms
Remove render-blocking requests
Shorten critical request chains
Use efficient cache lifetimes for static files
Est savings of 19 KiB
Ship less legacy JavaScript
Est savings of 10 KiB

Lighthouse 13.5.0 · mobile emulation (mid-range phone, slow 4G) from our US server. Lab data: real visitors' numbers vary.

How it works

  1. Google Lighthouse 13 runs in headless Chrome on our test server in Virginia, USA, with its default mobile settings.
  2. The page loads on a 412×823 phone screen (Lighthouse's Moto G Power profile) with simulated slow 4G (150 ms round trip, 1.6 Mbps) and a 4× CPU slowdown.
  3. Lighthouse waits up to 45 seconds for the page to load, and a full run typically takes under a minute.
  4. The URL must resolve to public IP addresses, and plain-HTTP requests made during the run pass through a network guard that blocks private and internal addresses.
  5. The results are lab data from a single run, so real visitors on other devices and networks will see different numbers.

How it is scored

The test score is Lighthouse's own mobile performance score, a weighted blend of Total Blocking Time (30%), Largest Contentful Paint (25%), Cumulative Layout Shift (25%), First Contentful Paint (10%) and Speed Index (10%). Grades follow our scale (A+ 95+, A 90+, B 80+, C 70+, D 60+, F below 60); if Lighthouse cannot calculate a score, our item-weighted formula is used instead.

CheckGoodNeeds workPoor
Performance score90–10050–89Below 50
Largest Contentful Paint (LCP)2.5 s or less2.5–4.0 sOver 4.0 s
Total Blocking Time (TBT)200 ms or less200–600 msOver 600 ms
Cumulative Layout Shift (CLS)0.1 or less0.1–0.25Over 0.25
First Contentful Paint (FCP)1.8 s or less1.8–3.0 sOver 3.0 s
Speed Index3.4 s or less3.4–5.8 sOver 5.8 s
Total page weight2 MB or less2–5 MBOver 5 MB

Grades: A+ 95–100 · A 90–94 · B 80–89 · C 70–79 · D 60–69 · F below 60.

FAQ

What are Core Web Vitals?
Core Web Vitals are Google's user-experience metrics for loading (Largest Contentful Paint), responsiveness (Interaction to Next Paint) and visual stability (Cumulative Layout Shift), and Google uses them as part of its page experience signals in search. A lab test cannot measure real interactions, so Lighthouse reports Total Blocking Time as the stand-in for INP.
What is a good Lighthouse performance score?
Lighthouse treats 90–100 as good, 50–89 as needing improvement and below 50 as poor, and this test uses the same bands. The score is measured on a throttled mid-range phone, so it is typically lower than a desktop score for the same page.
Why is my score different from PageSpeed Insights?
Lighthouse scores vary from run to run with server load, network routes and third-party scripts, and PageSpeed Insights runs from Google's own servers. This test runs Lighthouse 13 from our server in Virginia, USA, with the default mobile settings. PageSpeed Insights can also show field data from real Chrome users, which is a different measurement.
How do I improve Largest Contentful Paint?
Compress the main image (WebP or AVIF), give it preload and fetchpriority="high", and do not lazy-load it. Reduce render-blocking CSS and JavaScript and bring down your server response time. The report lists the opportunities with the largest estimated savings for your page.
Does this test measure desktop performance?
No. It uses Lighthouse's mobile settings, with a phone-sized screen, simulated slow 4G and a 4× slower CPU. For real-browser load times measured from 8 regions, run the global speed test.