Total Blocking Time
Definition
Total Blocking Time (TBT) is a key performance metric that quantifies the time a user cannot interact with a web page due to long-running JavaScript tasks. Specifically, it measures the period from when the first content is rendered on the screen until the main thread is free to respond to user inputs, such as clicks or keystrokes. This metric is crucial for understanding how delays in interactivity can affect user experience and satisfaction.
As user expectations for fast and responsive web experiences continue to rise, TBT has emerged as an essential indicator of performance. High TBT values can lead to frustrated users, potentially increasing bounce rates and negatively influencing conversions. By monitoring and optimizing TBT, UX designers and developers can ensure that users have a smooth and seamless experience when interacting with their web applications.
In practice, TBT is closely related to other performance metrics, such as First Contentful Paint (FCP) and Time to Interactive (TTI). While FCP measures how quickly content is displayed, TBT focuses on how long it takes for the page to become fully interactive. Understanding the relationship between these metrics helps teams make informed decisions about performance optimizations.
Expanded Definition
The concept of Total Blocking Time was introduced as part of Google's Web Vitals initiative, which aims to provide a standardized way to measure user experience on the web. TBT is particularly important because it highlights moments when users are unable to engage with a page, shedding light on potential points of frustration. By focusing on these blocking periods, web developers can identify and mitigate performance bottlenecks effectively.
Historically, many performance metrics did not account for interactivity and user engagement adequately. TBT fills this gap by providing a clear metric that emphasizes the importance of responsiveness. As web applications become increasingly complex, the need to monitor TBT and similar metrics becomes even more critical to ensure an optimal user experience.
Key Activities
Measuring TBT using performance monitoring tools.
Identifying long-running JavaScript tasks that contribute to high TBT.
Optimizing code to reduce blocking time.
Conducting user testing to assess the impact of TBT on user satisfaction.
Implementing best practices for asynchronous loading of resources.
Benefits
Improved user satisfaction by providing a responsive experience.
Reduced bounce rates due to faster interactivity.
Enhanced conversion rates as users can engage more effectively with the application.
Better performance scores in search engine rankings.
Informed decision-making regarding performance optimizations.
Example
For instance, an e-commerce website with a high TBT may prevent users from clicking on product images or adding items to their cart during the initial loading phase. If a user attempts to interact with the page but experiences delays, they may abandon their shopping session. By analyzing and optimizing TBT, the e-commerce site can ensure that users can interact with the site as soon as the content loads, thus improving overall satisfaction and sales.
Use Cases
Evaluating the performance of single-page applications (SPAs).
Assessing the impact of third-party scripts on web page responsiveness.
Optimizing web applications for mobile devices where users expect quick interactions.
Benchmarking against competitors to identify performance gaps.
Improving user experience in applications with complex rendering processes.
Challenges & Limitations
Difficulty in pinpointing the exact causes of long blocking times.
Trade-offs between functionality and performance due to complex JavaScript.
Variability in performance based on user devices and network conditions.
Limited awareness and understanding of TBT among some development teams.
Tools & Methods
Google PageSpeed Insights for measuring TBT.
WebPageTest for detailed performance analysis.
Lighthouse audits to identify blocking resources.
Chrome DevTools for real-time performance monitoring.
Performance budget planning tools to set TBT goals.
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UX Glossary. (2025, February 14, 2026). Total Blocking Time. UX Glossary. https://www.uxglossary.com/glossary/total-blocking-time
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