Free Total Blocking Time test

Measure Total Blocking Time and see what is keeping the main thread busy.

Run a controlled browser test for TBT, then inspect JavaScript CPU, third-party scripts and blocking resources from the same page load.

Free · No signup required · Real Chromium lab test

What this tool measures

TBT plus the browser work behind it

Total Blocking Time

Measures the time between FCP and load where long main-thread tasks block responsiveness in this lab run.

JavaScript CPU

Shows sampled main-thread CPU attributed to JavaScript during the current browser check.

Third-party JavaScript CPU

Separates third-party script cost when analytics, ads, widgets or external libraries consume browser time.

Top scripts

Ranks the scripts SiteState sees consuming the most sampled JavaScript CPU.

Render blockers

Surfaces blocking resources that can delay rendering before the page becomes visually useful.

LCP context

Keeps TBT beside Largest Contentful Paint so blocking work is not interpreted in isolation.

Interpret the result

High TBT usually means long tasks are occupying the main thread.

JavaScript execution, parsing, hydration and other browser work can create long tasks that prevent the page from responding promptly. Total Blocking Time summarizes that blocking during a lab page load.

TBT is useful for debugging, but it is not INP. SiteState keeps those concepts separate and uses JavaScript attribution to help identify which scripts deserve investigation first.

Methodology

SiteState measures TBT during a controlled Chromium lab run and combines it with sampled JavaScript CPU attribution. TBT is a lab diagnostic and is not presented as real-user INP.

FAQ

Common questions about this speed test

What is Total Blocking Time?

Total Blocking Time measures the time after First Contentful Paint where long main-thread tasks block the browser for more than 50 milliseconds.

What causes high TBT?

Common causes include heavy JavaScript execution, large bundles, hydration work and third-party scripts. SiteState also shows sampled CPU attribution to help narrow the investigation.