TBT (Total Blocking Time)
Sitecheck Team
A lab metric estimating how much the main thread is blocked during load.
TBT (Total Blocking Time) sums the time when the main thread is blocked by long tasks between First Contentful Paint and Time to Interactive. It is a lab-only metric used as a proxy for responsiveness.
Why it matters: High TBT indicates heavy JavaScript or layout work that delays interactions.
Quick tips:
- Split long tasks and reduce third-party scripts.
- Use code-splitting and lazy loading.
- Audit heavy framework hydration work.