This translation has not been editorially reviewed yet. The German version is authoritative. Deutsche Fassung →
Checking blocking time: when the main thread won't accept input
Total Blocking Time adds up the time during loading when the main thread is so busy it can't accept input. It's measured in the lab, and there it's the most reliable approximation of what users later experience as sluggishness.
- The cause is long, unbroken script execution. Once it's broken into smaller chunks, the browser can respond to input in between — the total work stays the same, but blocking time drops.
- Blocking time is the value that differs most clearly between mobile and desktop: the same work takes many times longer on a phone.
FAQ
Why doesn't TBT show up in field data?
Because it can't be measured that way in real visitors' browsers. In the field, INP takes over this role; blocking time is the lab value for it.
How do you lower blocking time?
Run less script, run it later, or break it into smaller chunks. The most effective step is usually checking third-party scripts: each one brings work along that nobody ordered.