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Core Web Vitals Checker & Comparison

Measure LCP, CLS, and INP using Google PageSpeed Insights. Check a single URL or compare two pages side by side.

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Core Web Vitals Checker — Measure & Compare Website Performance

Core Web Vitals are the performance metrics Google uses to evaluate real user experience on your website. Since 2021 they have been an official ranking signal, meaning slow pages can directly hurt your search visibility. Our free checker helps you measure and understand these metrics in seconds. For a full technical SEO report, run our SEO audit.

The tool uses the Google PageSpeed Insights API to run a full Lighthouse analysis and, when available, surfaces real-world Chrome User Experience Report (CrUX) data showing how actual visitors experience your page.

Understanding Every Metric

Each metric in the report tells you something different about how users experience your page. Here's what they mean in plain language and how to read the scores:

LCP — Largest Contentful Paint

How long until the main content of the page is visible. Think of it as the moment the page "feels loaded" to a visitor. Images, hero banners, and large text blocks are the most common LCP elements.

Thresholds: Good ≤ 2.5 s Needs Work ≤ 4.0 s Poor > 4.0 s
How to improve: Optimize images, use a CDN, reduce server response time, and eliminate render-blocking resources.

CLS — Cumulative Layout Shift

How much the page content jumps around while loading. High CLS means buttons and text shift unexpectedly — frustrating for users who accidentally tap the wrong element.

Thresholds: Good ≤ 0.1 Needs Work ≤ 0.25 Poor > 0.25
How to improve: Always set width/height on images and videos, reserve space for ads and embeds, avoid inserting content above existing content.

TBT — Total Blocking Time (proxy for INP)

How long the page is frozen and cannot respond to taps or clicks. In lab testing this serves as a proxy for the real-world INP (Interaction to Next Paint) metric that Google uses for ranking.

Thresholds: Good ≤ 200 ms Needs Work ≤ 600 ms Poor > 600 ms
How to improve: Reduce JavaScript execution time, break up long tasks, defer non-critical scripts, and remove unused code.

FCP — First Contentful Paint

How long until the first piece of content (text or image) appears on screen — even before the full page loads. It's the user's first signal that the page is working.

Thresholds: Good ≤ 1.8 s Needs Work ≤ 3.0 s Poor > 3.0 s
How to improve: Eliminate render-blocking resources, inline critical CSS, use font-display: swap for web fonts.

SI — Speed Index

How quickly the visible parts of the page are filled in. A lower score means content appears faster for the user rather than all at once at the end.

Thresholds: Good ≤ 3.4 s Needs Work ≤ 5.8 s Poor > 5.8 s
How to improve: Minimize main-thread work, reduce JavaScript payload, and optimize the critical rendering path.

Why Comparing URLs Matters

Raw numbers alone don't tell you whether your performance is competitive. By entering a competitor's URL alongside your own, you get a clear picture of where you lag behind and where you're ahead. The comparison table highlights the winner for each metric so you can prioritize improvements that close the gap.

Lab vs. Field Data

Lab data (Lighthouse) runs in a simulated environment — great for debugging and reproducibility. Field data (CrUX) reflects what real Chrome users experience on the page, measured at the 75th percentile over 28 days. Google's ranking algorithm uses field data, so that's the ultimate benchmark.

Combined with pre-rendering for JavaScript sites, optimizing Core Web Vitals ensures search engines can both crawl and rank your pages effectively — leading to better indexation and higher organic traffic.

Frequently Asked Questions

What are Core Web Vitals?
Core Web Vitals are a set of three user-centric performance metrics defined by Google: Largest Contentful Paint (LCP) measures loading speed, Cumulative Layout Shift (CLS) measures visual stability, and Interaction to Next Paint (INP) measures responsiveness. Since 2021, they are part of Google's ranking signals for search results.
What is LCP (Largest Contentful Paint)?
LCP measures how long it takes for the largest visible content element (image, video, or text block) to render on screen. A good LCP is 2.5 seconds or less. Poor LCP is often caused by slow server response, render-blocking resources, slow resource load times, or client-side rendering.
What is CLS (Cumulative Layout Shift)?
CLS measures how much the page layout shifts unexpectedly while content is loading. A good CLS score is 0.1 or less. Common causes of high CLS include images without dimensions, dynamically injected content, web fonts causing FOIT/FOUT, and ads or embeds without reserved space.
What is INP (Interaction to Next Paint)?
INP (Interaction to Next Paint) replaced FID as a Core Web Vital in March 2024. It measures the latency of all user interactions (clicks, taps, keyboard) throughout the page lifecycle, reporting the worst interaction. A good INP is 200 milliseconds or less. In lab testing, Total Blocking Time (TBT) serves as a proxy for INP.
What is the difference between Lab data and Field data?
Lab data comes from Lighthouse simulating a page load in a controlled environment — it's reproducible but doesn't reflect real user conditions. Field data (CrUX — Chrome User Experience Report) is collected from real Chrome users over 28 days. Field data shows the 75th percentile of actual user experiences and is what Google uses for ranking.
How does the comparison feature work?
Enter two URLs — your page and a competitor's — and both are analyzed in parallel via PageSpeed Insights. The tool generates a side-by-side comparison table for all key metrics (LCP, CLS, TBT, FCP, Speed Index) with a clear winner indicator. If CrUX field data is available for both URLs, real-world metrics including INP are also compared.
Why is my INP not showing in lab data?
INP requires real user interactions to measure, so it cannot be fully simulated in a lab environment. In the Lab Metrics section, Total Blocking Time (TBT) is shown as a proxy — it correlates well with INP. Real INP values appear in the "Real-World Data (CrUX)" section if enough Chrome users have visited the page.
Is this tool free to use?
Yes, the Core Web Vitals Checker is completely free with no registration required. It uses the publicly available Google PageSpeed Insights API to analyze pages.