JavaScript Performance · AuditJet Glossary
Long Task
A JavaScript task that takes longer than 50ms to complete and blocks the browser's main thread, preventing user interaction.
Long tasks are the primary cause of high INP and TBT. When a long task is running, the browser cannot respond to user input (clicks, taps, key presses) until the task completes. Chrome DevTools and Lighthouse identify long tasks via the Performance panel and the Long Tasks API. Breaking long tasks into smaller chunks with scheduler.yield(), setTimeout(), or requestIdleCallback() is the primary fix.
Related terms
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.
The single thread in a browser where JavaScript execution, style calculations, layout, painting, and compositing occur.
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