TBT
TBT is a lab metric measuring how long the main thread is blocked by long tasks. It’s useful for diagnosing INP/FID issues.
Definition
TBT (Total Blocking Time) is a lab metric from Lighthouse/PageSpeed that measures how long the main thread is blocked by long tasks during load, preventing quick interaction responses. It’s not a Core Web Vitals metric but is helpful for diagnosing responsiveness problems.
Why it matters
- Helps identify main-thread blocking long tasks
- A useful lab proxy when improving INP
- Highlights JS that should be split or deferred
How to implement
- Break up long tasks (chunk work, schedule wisely)
- Reduce third-party scripts and synchronous computations
- Split bundles and defer non-critical loading
FAQ
Common questions about this term.