• Data Source: Page Speed Insights
  • Type: Numeric (Time, Score)


Total Blocking Time (TBT) measures the total amount of time between First Contentful Paint (FCP) and Time to Interactive (TTI) where the main thread is blocked for long enough to prevent input responsiveness. It captures the periods when the browser is busy with tasks, preventing the user from interacting with the page. Page Speed Insights categorizes TBT scores into three groups:

  • Good TBT Score (Green): TBT of 200 milliseconds or less.
  • Needs Improvement (Orange): TBT between 200 and 600 milliseconds.
  • Poor TBT Score (Red): TBT longer than 600 milliseconds.

TBT is an important performance metric because it reflects the page’s overall responsiveness and interactivity, crucial for user experience.

Use Cases

  1. User Interaction Optimization: Reduce TBT to ensure that the page remains responsive and interactive, improving the overall user experience and reducing frustration.
  2. Performance Monitoring: Regularly monitor TBT using Page Speed Insights to identify and resolve issues causing long main thread blocking times, ensuring consistent performance.
  3. Web Development Prioritization: Focus development efforts on reducing long tasks and optimizing the main thread, such as by deferring non-critical scripts, breaking up long tasks, and minimizing JavaScript execution times.
  4. SEO Enhancement: Improve TBT to enhance overall page performance, contributing to better search engine rankings as search engines prioritize pages with better user experiences.
  5. Conversion Rate Improvement: Increase conversion rates by ensuring that the page remains responsive throughout the loading process, encouraging users to interact and engage with the content.


  • Positive Indicator: A low TBT score (200 milliseconds or less) indicates that the page is responsive and interactive, contributing to a positive user experience and potentially higher search engine rankings.
  • Negative Indicator: A high TBT score (longer than 600 milliseconds) suggests that the page experiences significant blocking times, leading to poor responsiveness, higher bounce rates, lower user satisfaction, and reduced SEO effectiveness. A TBT between 200 and 600 milliseconds indicates room for improvement to enhance page interactivity and responsiveness.

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