Free JavaScript Rendering Lighthouse Test
Identify render-blocking scripts, unused JavaScript bundles, and long JS execution tasks that delay page load and hurt your Lighthouse Performance score.
Free JavaScript Rendering Lighthouse Test
JavaScript is the number one cause of poor Lighthouse Performance scores. Our free test uses Google Lighthouse to find every render-blocking script, unused JS bundle, and long main-thread task slowing down your Critical Rendering Path.
How JavaScript Destroys Lighthouse Performance Scores
Every synchronous script tag in your HTML is a parking brake on the browser's Critical Rendering Path. The browser must download, parse, compile, and execute each script before it can continue rendering HTML. On a throttled mobile connection, a 200KB JavaScript bundle can add 3 seconds of render-blocking time — pushing your LCP well above Google's 2.5-second "Good" threshold. Our free Lighthouse test surfaces these culprits by name and byte size.
Unused JavaScript is equally damaging to your Total Blocking Time score. If your JS bundle includes 400KB of code but only 60KB is executed on the initial page load, you're forcing the browser to parse and compile dead code. Code-splitting with dynamic imports and proper tree-shaking can reduce bundle size by 60 to 80 percent. Our Lighthouse audit identifies which scripts have the highest unused bytes ratio so you know exactly where to start.