Total Blocking Time
Also called: TBT
Total Blocking Time (TBT) is a lab performance metric that measures the total time after First Contentful Paint during which the main thread was blocked long enough to prevent input responsiveness. It is Lighthouse's heaviest-weighted metric and the best lab proxy for the field metric INP.
Total Blocking Time sums the “blocking” portion of every long task between First Contentful Paint and the point the page becomes interactive. A long task is any main-thread task over 50 ms, and the blocking part is everything past that 50 ms mark. So a task that runs for 120 ms adds 70 ms of blocking time, and TBT is the total across all such tasks.
Thresholds and where it fits
On average mobile hardware, aim for under 200 ms. web.dev grades 200 ms or less as good, 200 to 600 ms as needs improvement, and over 600 ms as poor. TBT is not itself a Core Web Vital. It is a lab metric, and it carries 30% of the Lighthouse performance score in Lighthouse 10, the single largest weight. Its job is to predict INP (Interaction to Next Paint), the field metric Google actually collects. A low TBT usually means a low INP, though the two can diverge because TBT flags potential problems that real users may never trigger.
The usual culprits are heavy JavaScript: large bundles, framework hydration, and third-party tags. Fixes target the main thread directly. Ship less JS through code splitting, move work to web workers, and break long functions into smaller tasks that yield to the main thread (for example with scheduler.yield()).
How it affects your traffic
TBT is not a ranking signal on its own, but it is the lab warning light for INP, which is a Core Web Vital. When Lighthouse TBT is high, real users likely feel taps and clicks lag, which hurts conversions and drags the INP that feeds Google's page experience signals. A technical SEO pass profiles the long tasks, trims and defers the JavaScript causing them, and re-tests until TBT clears the 200 ms bar. That is the exact work our Technical SEO service handles, from bundle audits to third-party script cleanup.
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