MetricsTBT
Total Blocking Time
The total time the main thread was blocked by long tasks between first paint and interactivity, measured in the lab.
- Good
- <= 200 ms
- Needs work
- 200 to 600 ms
- Poor
- > 600 ms
Lab data (Lighthouse)
TBT is a lab metric and the closest lab proxy for INP. It sums the blocking portion (everything over 50 ms) of every long task after FCP, showing how much a page would ignore input while scripts run.
Because it is measured in a controlled lab run, TBT is stable and great for catching regressions in CI before they reach real users as a poor INP.
What moves it
- Large JavaScript bundles parsed and executed at load.
- Expensive hydration in client-rendered frameworks.
- Third-party tags running heavy work on the main thread.
Related terms
- CWVCore Web VitalsGoogle's set of three field metrics for loading (LCP), interactivity (INP) and visual stability (CLS) of a page.
- LCPLargest Contentful PaintThe time until the largest visible element in the viewport (image, video or text block) has rendered.
- INPInteraction to Next PaintHow long the page takes to visually respond after a user interacts, measured across the whole visit.
- CLSCumulative Layout ShiftA unitless score for how much visible content unexpectedly moves around while the page loads and runs.
- FCPFirst Contentful PaintThe moment the browser renders the first piece of content: text, an image or a canvas.
- TTFBTime to First ByteThe time from the start of navigation until the browser receives the first byte of the response.
- FIDFirst Input Delay (retired)The delay between a visitor's first interaction and the browser starting to process it. Replaced by INP.
- SISpeed IndexA lab metric for how quickly the visible parts of a page fill in during load, based on video capture.
- TTITime to Interactive (legacy)A lab metric for when a page has rendered and can reliably respond to input quickly.
- Page LoadThe classic load event: everything on the initial page, including images and subresources, has finished loading.
- Route LoadThe loading time of an in-app navigation in a single-page app, where no full page reload happens.
- Long Tasks & LoAFAny piece of JavaScript that occupies the main thread for more than 50 ms, blocking the page from responding.
- Experience ScoreA single 0 to 10 number fastmon derives from LCP, INP, CLS, FCP, TTFB, load time and error rate.
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