TL;DR
Main thread work is the cumulative time the browser's primary thread spends parsing HTML, evaluating and executing JavaScript, performing style and layout, and painting, before it can respond to interactions.
What is Main Thread Work?
Main thread work is the cumulative time the browser's primary thread spends parsing HTML, evaluating and executing JavaScript, performing style and layout, and painting, before it can respond to interactions.
Learn more in our how to improve main thread work guide, or work with us through our done-for-you speed optimization service.
How Main Thread Work is Measured
Lighthouse reports total main thread work and a breakdown by activity category from a recorded trace.