TBT (Total Blocking Time)
The total time the browser's main thread was blocked for more than 50ms at a stretch, between FCP and the page becoming interactive, unable to respond to a user interaction.
Updated: Dec 2025
Computational logic
Every processing block longer than 50ms between FCP and the page becoming fully interactive is identified, and only the portion beyond that 50ms is added to the total; under 200ms is good and beyond 600ms is poor. This metric can only be measured in this lab test.
Use-case narrative & business value
This metric is the engineering team's early-warning, reliable lookout for predicting responsiveness problems before they hit the real world, since TBT is the lab-test counterpart of INP. Because this measurement runs in a lab environment, it predicts that same risk in advance by measuring how occupied the main thread is: a page with high TBT is very likely to also score poorly on INP once it hits real traffic.
From a business angle, this metric lets the product team find the root of processing blockages and head off customer churn and conversion-rate drops before shipping changes — and before real users ever run into a slow, freezing page.
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