Total Blocking Time (TBT)
Total Blocking Time (TBT) measures the total 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 measures the total amount of time your webpage was blocked, preventing the user from interacting with your page and it is a replacement for the field-only First Input Delay (FID) metric.
A good user experience is 150ms or less.
