JavaScript execution time
JavaScript execution time is how long the browser spent on the page’s scripts during Google’s lab test: reading them, preparing them to run, and running them. Lighthouse lists the time per script, so you can see which files cost the most.
Why it matters
Section titled “Why it matters”While the browser works on a script, it can’t do anything else on the page. It doesn’t draw new content and doesn’t react to taps. A page with a lot of JavaScript can look finished but still feel sluggish, and the work shows up in Total Blocking Time and, for real visitors, in Interaction to Next Paint.
Phones feel this most. The lab test simulates a phone with a slower processor than a typical desktop computer, so the same scripts take several times longer there.
The value itself doesn’t count toward the Performance score. It explains why some of the metrics that do count are high.
What influences it
Section titled “What influences it”WordPress: The most common cause of a long JavaScript execution time on WordPress sites is the number of scripts that themes and plugins load on every page. Page builders, sliders, animation libraries, form plugins, cookie notices and shop features each bring their own code, often on pages where they aren’t used.
Browser: Scripts from other services often weigh the most: tag managers, analytics, ads, chat widgets, embedded videos and social media feeds. Their size and how much work they do as soon as they load decide the time.
Cache: A browser that has a script cached skips the download, and it can reuse some of the preparation. Running the script still takes the same time. The lab test always starts with an empty cache.
In Permo
Section titled “In Permo”Permo shows the JavaScript execution time on its PageSpeed page under Processing, from Google’s lab test.
Reference values
Section titled “Reference values”Lighthouse shows a warning when JavaScript execution takes longer than 2 seconds and fails the check above 3.5 seconds (source: Chrome for Developers, Reduce JavaScript execution time, accessed October 16, 2026).