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.
Frequently asked questions
What is Main Thread?
The single thread in a browser where JavaScript execution, style calculations, layout, painting, and compositing occur.
How does AuditJet measure Main Thread?
AuditJet runs automated Google Lighthouse audits that surface Main Thread scores alongside actionable fix guidance. You can track Main Thread trends across all your pages and receive alerts when the metric regresses.
Monitor Main continuously
AuditJet tracks Core Web Vitals on a schedule with revenue impact alerts — powered by Google Lighthouse and Google PageSpeed Insights.
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