Total Blocking Time

The total time that the browser is blocked from handling user inputs.

Total Blocking Time (TBT) measures the total amount of time, during a complete page load, during which the web browser cannot respond to a user input because the main thread is blocked. The main culprit for having a poor TBT time is excessive amounts of JavaScript being loaded and run while the page loads. To improve this score, a website could defer unnecessary JavaScript so that it loads and executes lazily after the main page loads. Another opportunity to improve this score is to optimize existing JavaScript so it runs faster, or uses a Web Worker thread to avoid blocking teh main thread.

A TBT under 200ms is considered to be fast and a TBT over 600ms is slow.

For more information, please read the documentation on Web.dev.