Main thread

The main thread is the single lane of work where a browser builds the page, runs most JavaScript, and responds to clicks, taps and scrolling. Only one task can run on it at a time, so while a long task runs, the page appears frozen. Performance advice often focuses on keeping the main thread free.

This is exactly what INP measures: how long the main thread makes a user wait between an interaction and the next visual update.

Heavy event handlers, large third-party scripts and expensive rendering work are the usual causes of a busy main thread.

Learn more

• Main thread - MDN glossary entry
• JavaScript
• INP