Making Sense of the Performance Extensibility API

August 4, 2025

Google Chrome DevTools Performance tab showing a timeline of page loading events, with a highlighted section for "cssTime" indicating external CSS was fetched and parsed in 1.17 seconds, and details such as file size and download time displayed below.

Google Chrome recently introduced the Performance Extensibility API, a means of putting your code front-and-centre in Chrome’s Performance panel. Their own documentation is somewhat dry and doesn’t really state the benefits or outcomes very clearly, so I spent a couple of hours looking into it and here are the key takeaways.

Source: Making Sense of the Performance Extensibility API – CSS Wizardry

While it’s Chrome only (for now ?), the Performance Extensibility API provides ways for developers to get more fine grained understanding of the performance of a page. Harry Robert provides the details.