Documentation · Web Vitals

Installation

Open the extension page on the Chrome Web Store (it also works on Edge and other Chromium-based browsers).
Click Add and confirm the permissions.
Pin the Web Vitals icon to your toolbar.

You can also load it unpacked from chrome://extensions (developer mode → Load unpacked) if you’re working with the repository code.

Usage

Visit any website and click the Web Vitals icon. The popup is organised into six tabs:

Web Vitals — LCP, CLS, INP, FCP and TTFB in real time, colour-coded by threshold and compared against real-user data (CrUX). When a metric needs improvement or is poor, concrete suggestions appear (for example, preloading the main image or reserving space to avoid layout shifts).
Server Info — IP, geographic location, ISP, organisation, ASN, time zone and server hostname.
DNS Info — A, AAAA, MX, NS, TXT, SOA and CNAME records, plus detected subdomains.
SEO — a 0–100 score (title, meta description, headings, images, structured data, Open Graph, security headers), a domain authority estimate, and suggestions for each issue.
Tech Stack — CMS and generators, server software, JS frameworks and libraries, analytics tools and font providers.
A11y — an accessibility score with an estimated WCAG level (A, AA, AAA) and detection of common issues (missing alt text, empty links or buttons, unlabelled fields, heading skips, etc.).

Metrics: how to read them

Good (green) — the page is above the recommended threshold.
Needs improvement (orange) — close, but not optimal.
Poor (red) — it doesn’t meet the threshold and needs attention.

Optimization guides: LCP · CLS · INP.

Export report

Click the download icon in the popup header to export a full Markdown report with the data from every tab. Tabs you haven’t opened are loaded automatically before exporting, so the report is never incomplete.

Extras

Light/dark mode, with your preference saved between sessions.
Chrome side panel (requires Chrome 114 or higher): click the side panel icon in the header.

Privacy

Performance analysis runs locally, in your browser. The extension does not send your personal data, your history or your metrics to VANTAG servers, and it doesn’t track you or collect analytics.

To fetch data that isn’t available in the browser, it does query public third-party APIs: Chrome UX Report (CrUX) for field data, ipwhois for IP geolocation, Google Public DNS for DNS records, Mozilla HTTP Observatory for security headers, and RDAP for domain registration data. None of them requires a user key or sign-up.