Shining a Light on JavaScript Performance with Lighthouse

Unsure how much JavaScript overhead affects your users? Lighthouse provides a JavaScript execution time audit that quantifies the overall impact of JavaScript on page‑load performance.

The audit appears in Chrome DevTools’ Audits panel and can also be accessed via WebPageTest.

For the example site, a mid‑range mobile device processes the core bundle in about 51 seconds, including network transfer, meaning users can interact with the page in under a minute.

This time includes parsing, compiling, and executing scripts on a typical mid‑tier mobile device. Sites like dev.to keep their main bundle lightweight, minimizing script dependencies.

To control JavaScript cost, only transmit the code the user actually needs and lazily load the rest using techniques such as code‑splitting. The author demonstrates this with Chrome DevTools’ Code Coverage feature.

Recording a coverage session shows that roughly 57% of the loaded code may not be needed upfront, making it a prime candidate for on‑demand loading.

Lighthouse also offers handy utilities such as checking whether scripts are properly minified or compressed.

For headless Chrome automation, Puppeteer includes a code‑coverage example that visualizes JavaScript coverage during page load.

In summary, JavaScript can dramatically affect user experience; Lighthouse helps you keep transmission and processing time low by sending only required code, minifying/compressing scripts, and removing unused code and dependencies.