Total Blocking Time (TBT)
Total Blocking Time measures the total time the main thread was blocked, preventing response to user input. SaaSalyst checks your TBT score via Google PageSpeed Insights to assess interactive responsiveness.
What SaaSalyst Checks
SaaSalyst uses the Google PageSpeed Insights API to measure Total Blocking Time on mobile. TBT quantifies how long the main thread was occupied by long tasks (>50ms) during page load, directly impacting perceived interactivity.
Why This Matters
High TBT means users cannot interact with your page during load — clicks, taps, and keystrokes are ignored. For SaaS products, this creates a poor first impression.
Google uses TBT as a Lighthouse metric that correlates with real-world responsiveness. Lower TBT means your app feels snappier and more professional.
How to Fix It
- Reduce JavaScript execution time by code-splitting and lazy-loading non-critical scripts.
- Break up long tasks into smaller async chunks using requestIdleCallback or setTimeout.
- Remove unused JavaScript and third-party scripts that block the main thread.
Frequently Asked Questions
What is a good Total Blocking Time?
SaaSalyst rates TBT as pass at 200ms or below, warn between 200-600ms, and fail above 600ms, based on Google's recommended thresholds.
How does TBT affect my Business Readiness Score?
SaaSalyst rates TBT as medium severity in the Performance category. High TBT indicates your app may feel unresponsive to users.
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