Overview
TBT quantifies the severity of a page's non-responsiveness during the loading process. It is calculated by summing the 'blocking' portion of all long tasks (tasks that take longer than 50ms) between First Contentful Paint (FCP) and Time to Interactive (TTI).
Calculation Example
If a task takes 70ms, the blocking portion is 20ms (70ms - 50ms). TBT is the sum of these blocking portions.
Importance
TBT is a strong proxy for Interaction to Next Paint (INP) in a lab environment. Reducing TBT is critical for ensuring a smooth, responsive user experience during page load.