Free website JavaScript performance test

See which JavaScript is consuming browser CPU on your website.

Run a real-browser page load and inspect sampled JavaScript CPU, third-party impact, Total Blocking Time and the heaviest scripts SiteState can attribute.

Free · No signup required · Real Chromium lab test

What this tool measures

Website JavaScript impact, not a code-snippet benchmark

Sampled JavaScript CPU

Measures browser CPU time SiteState can attribute to JavaScript during the page load.

Third-party JavaScript

Separates externally hosted script CPU so analytics, ads, widgets and embeds can be evaluated independently.

Top script attribution

Shows the scripts using the most sampled JavaScript CPU in the current run.

Estimated blocking impact

Adds blocking estimates where SiteState can relate JavaScript work to main-thread delay.

Total Blocking Time

Shows the broader long-task blocking signal from the same controlled browser run.

Page-weight context

Keeps CPU findings beside transfer size and resource counts so network and execution cost can be separated.

Interpret the result

This tool measures JavaScript impact on a webpage, not raw language speed.

Generic JavaScript benchmark tools compare code snippets or browser engines. SiteState answers a different question: how much JavaScript work does this actual website ask the browser to do during page load?

That distinction matters because a fast JavaScript engine cannot compensate for excessive bundles, tag-manager payloads, third-party widgets or long-running application code.

Methodology

SiteState loads the page in Chromium and samples JavaScript main-thread work during the synthetic run. CPU attribution is diagnostic evidence from this page load, not a universal benchmark of the JavaScript language or the visitor's device.

FAQ

Common questions about this speed test

What does this JavaScript performance test measure?

It measures JavaScript impact on a webpage, including sampled browser CPU, third-party JavaScript CPU, top scripts and blocking context from a controlled page load.

Is this a JavaScript code benchmark?

No. This tool does not benchmark code snippets or compare JavaScript engines. It analyzes the JavaScript work performed while loading a real website.

Why are third-party scripts important?

Analytics, advertising, chat, video and other third-party scripts can add network and main-thread cost that the website owner may not directly control.