Total Blocking Time (TBT) is a lab performance metric that measures the total duration of time between First Contentful Paint and Time to Interactive during which the main thread was blocked long enough to prevent input responsiveness. A task is considered a “long task” if it runs for more than 50 milliseconds; TBT sums only the portions of those tasks that exceed that 50 ms threshold. For example, a 90 ms task contributes 40 ms to TBT. Google considers TBT under 200 ms as “good” in Lighthouse and PageSpeed Insights. TBT is a lab-only metric — it cannot be measured directly in the field — but it correlates strongly with the field metric INP and was historically used as a proxy for FID. High TBT is most commonly caused by large JavaScript bundles, excessive third-party scripts, unoptimised render-blocking resources, and synchronous DOM manipulation during page load.