Total Blocking Time
ELI5 — The Vibe Check
Total Blocking Time is the sum of all the moments your browser was too busy running JavaScript to notice you desperately clicking buttons. It's like being on hold — each second feels like ten, and you're seriously considering switching providers.
Real Talk
Total Blocking Time (TBT) measures the total time between First Contentful Paint and Time to Interactive where the main thread was blocked for more than 50ms. Each task's blocking time is the portion exceeding the 50ms threshold. Lower TBT means better interactivity.
When You'll Hear This
"Our TBT is 800ms — that third-party analytics script is destroying us" / "Lighthouse flags anything over 200ms TBT as needing improvement"
Related Terms
First Paint
First Paint is the moment your browser stops showing a blank white screen and puts literally anything on it.
Long Tasks API
The Long Tasks API is a snitch that tells you whenever JavaScript hogs the main thread for more than 50 milliseconds.
Time to Interactive
Time to Interactive is when your page stops being a beautiful painting you can't touch and actually starts responding to clicks.