Total Blocking Time (TBT)
A lab metric measuring the total time the main thread was blocked long enough to prevent input responsiveness.
Quick Definition
A lab metric measuring the total time the main thread was blocked long enough to prevent input responsiveness.
Full Explanation
Total Blocking Time (TBT) measures the total amount of time between First Contentful Paint (FCP) and Time to Interactive (TTI) where the main thread was blocked for long enough to prevent input responsiveness. TBT is a lab metric (not measured in the field) that correlates well with INP. A long task is any JavaScript task that runs for more than 50ms; the blocking time is the portion beyond 50ms. A good TBT is under 200ms. TBT is useful for identifying JavaScript performance issues during development. To improve TBT, break up long tasks, defer non-critical JavaScript, and reduce JavaScript execution time.