Total Blocking Time
A lab metric that sums how long the browser's main thread was blocked by long tasks after First Contentful Paint.
Definition
Total Blocking Time (TBT) is a lab performance metric that measures the total amount of time between First Contentful Paint and Time to Interactive during which the main thread was blocked long enough to prevent input responsiveness. It quantifies how much heavy work delays user interaction.
TBT counts 'long tasks' — main-thread tasks running longer than 50 milliseconds — and sums only the portion of each task that exceeds that 50 ms threshold. Because the browser cannot interrupt a task in progress, long tasks delay the response to user input. TBT is measured in the lab rather than from real users, and a low TBT often correlates with a low Interaction to Next Paint, so it serves as a useful proxy for responsiveness problems. A common target is less than 200 milliseconds on average mobile hardware.
Examples
Lighthouse diagnostics
A Lighthouse report shows a TBT of 600 ms, pointing to long JavaScript tasks that block the main thread during page load.
Responsiveness proxy
A developer reduces TBT in lab testing by splitting large scripts, expecting a corresponding improvement in field Interaction to Next Paint.
Sources
Related terms
- Interaction to Next PaintA Core Web Vitals metric measuring how long a page takes to visually respond to a user interaction, sampled across the page's lifetime.
- First Contentful PaintA web performance metric measuring the time from page load start to the first visible text, image, or non-blank canvas being rendered.
- Core Web VitalsA set of three Google metrics that measure real-world page experience: loading speed, interactivity and visual stability.
- LighthouseAn open-source automated tool from Google that audits a web page for performance, accessibility, SEO and other best practices.
- Render-Blocking ResourcesResources — typically CSS and synchronous JavaScript — the browser must fetch and parse before it can paint the first frame of a page.
- Page SpeedHow quickly a web page loads and becomes interactive, measured by metrics such as Largest Contentful Paint, Interaction to Next Paint, and Time to First Byte.
- First Input DelayA retired Core Web Vital that measured the delay between a user's first interaction and the browser starting to process it.
Where QueryCatch uses this
Last updated: 16/05/2026