Total Blocking Time (TBT)

How long JavaScript blocked the page from responding during load.

Good

200 ms or less

Poor

over 600 ms

What it measures

TBT adds up the blocking part of every task longer than 50 ms during load. It is the lab stand-in for INP.

Common causes of a poor TBT

• Large JavaScript bundles.
• Hydrating the whole app at once.
• Many third-party tags on load.

How to improve it

1. Split bundles by route.
2. Hydrate below-the-fold components lazily.
3. Load non-essential tags after the page is interactive.