Glossary term
What is main thread? Meaning for developers
The one browser thread that runs your JavaScript and builds every frame. Long work on it blocks clicks and painting.
Learn it properly · Scenario DebuggingLesson 1: “The API Is Fast but the UI Is Slow”: How to Solve ItThe network tab says 80 ms, but users wait two seconds. Long main-thread tasks, huge JSON, many re-renders, layout thrashing and hydration cost, all linked to INP.