Front-End’s Missing Metric: The TBT Window
Summary
The article explains Total Blocking Time (TBT) and introduces the TBT Window—the interval between First Contentful Paint and Time to Interactive during which TBT is counted. It discusses how the retirement of TTI from Lighthouse's score affects interpretation, and how changes to FCP/TTI can widen the TBT Window, causing apparent TBT regressions even without more blocking work. It uses a client case to illustrate that TBT spikes may be window artifacts, and recommends exposing the TBT Window in tooling and monitoring TBT alongside FCP and TTI.