JavaScript Performance · AuditJet Glossary
Main Thread
The single thread in a browser where JavaScript execution, style calculations, layout, painting, and compositing occur.
The browser's main thread is a single-threaded environment — only one task can run at a time. JavaScript execution, DOM manipulation, CSS calculations, and layout all compete for main thread time. Heavy JavaScript is the primary reason for a congested main thread, causing high TBT and INP scores. Web Workers can offload computation to background threads to keep the main thread free.
Related terms
A JavaScript task that takes longer than 50ms to complete and blocks the browser's main thread, preventing user interaction.
A Core Web Vital that assesses overall responsiveness by observing the latency of all qualifying interactions throughout the entire page lifecycle and reporting the worst-case value.
The total time between First Contentful Paint and Time to Interactive during which the main thread was blocked for long enough to prevent input responsiveness.
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