Script Execution Time

Script Execution Time is the duration the browser's JavaScript engine spends parsing, compiling, and running JavaScript code on a webpage.

Meaning & Context

This metric specifically measures CPU processing time rather than download time, representing how long JavaScript blocks the main thread from responding to user interactions. Script execution time is particularly impactful on mobile devices with slower processors and can cause pages to feel unresponsive even after visual content has loaded. The metric includes both initial execution and any subsequent JavaScript-triggered updates to the page.

Why It Matters

In A/B testing implementations, excessive script execution time from testing platforms or variant-specific JavaScript can delay interactivity and harm conversion rates independent of the actual changes being tested. This is especially problematic for client-side testing tools that must execute complex targeting logic and DOM manipulation. Monitoring script execution time helps teams identify when testing infrastructure or specific variants create performance bottlenecks that compromise the validity of test results.

Example

An A/B test on a product listing page shows both variants performing poorly compared to historical data; performance analysis reveals the new testing platform's targeting script requires 850ms of execution time, delaying Time to Interactive and causing a 12% increase in bounce rate across all test groups.