In the evolving world of web performance and user experience, Google’s Core Web Vitals (CWV) play a crucial role in determining how well a website performs, especially from the perspective of real-world user interactions. Among the three primary metrics that make up CWV—Largest Contentful Paint (LCP), First Input Delay (FID), and Cumulative Layout Shift (CLS)—there is also a related concept that webmasters and developers must understand: Total Blocking Time (TBT).
What is TBT?
Total Blocking Time (TBT) is a metric that measures the total amount of time a page is blocked from responding to user interactions due to JavaScript execution. This time is crucial because it directly influences how quickly a page can respond to a user’s input, such as clicks, taps, or typing. Essentially, TBT gauges the time between when a user tries to interact with a page and when the page is actually able to respond to that action.
TBT is particularly important because, if a page takes too long to process JavaScript tasks and delays user interactions, it can lead to frustration and a poor user experience. A low TBT means the page is responsive and users can interact with it smoothly. On the other hand, a high TBT indicates that the page is unresponsive and users will experience delays when trying to interact with the site.
Why is TBT Important?
The primary focus of TBT is user experience. A site that has a high TBT means that users may encounter delays in interacting with elements on the page, such as clicking buttons, filling out forms, or navigating links. This lack of responsiveness can lead to frustration, abandoned sessions, and ultimately, lower user engagement.
Google’s emphasis on Core Web Vitals, including TBT, reflects the growing recognition that performance metrics have a direct impact on user satisfaction, which in turn affects business outcomes such as conversion rates, bounce rates, and SEO rankings.
How TBT is Measured
TBT is calculated by summing up all periods where the main thread is blocked for more than 50 milliseconds during the page load process. These periods occur when long tasks (such as JavaScript execution or rendering processes) prevent the browser from responding to user inputs.
To give an example, imagine a user trying to click a button right after a page starts loading. If the main thread is busy running JavaScript or rendering content, the click may be ignored or delayed, which would increase the TBT. Measuring TBT helps web developers understand how many of these delays occur during the page load process and how long they last.
TBT in the Context of Core Web Vitals
While TBT is not one of the primary metrics in Google’s Core Web Vitals, it has a close relationship with FID (First Input Delay). FID measures the delay between a user’s first interaction with the page and the time the browser responds to that interaction. TBT, on the other hand, provides a more granular view of the blocking time during the page load phase, which ultimately contributes to the overall FID.
By optimizing TBT, developers can improve FID and, therefore, the overall user experience. This is particularly important because FID is one of the metrics Google uses to evaluate user experience for SEO purposes. If TBT is high, FID is likely to be high as well, which could negatively impact a site’s SEO performance.
Best Practices for Reducing TBT
Reducing TBT requires improving how the browser handles JavaScript execution and other tasks that block the main thread. Here are some strategies to lower TBT:
- Optimize JavaScript Execution:
- Break up long tasks into smaller chunks using techniques like async and defer attributes for scripts, or using requestIdleCallback to defer non-essential tasks until the browser is idle.
- Use Web Workers:
- Web workers allow JavaScript to run in the background, off the main thread. This reduces the load on the main thread and minimizes blocking.
- Optimize Resource Loading:
- Prioritize critical resources (e.g., CSS, JavaScript, images) to load first, and defer non-essential resources until after the page has loaded. Tools like lazy loading can help achieve this.
- Minimize Third-Party Scripts:
- Excessive third-party scripts (e.g., ads, trackers, widgets) can significantly increase TBT. Evaluate and limit the use of third-party scripts to only those that are essential for the site’s functionality.
- Code Splitting:
- Instead of loading large JavaScript files all at once, use code splitting to load only the code that is necessary for the current page, reducing the blocking time during the initial load.
- Reduce Main Thread Work:
- Optimize the JavaScript code to perform less work on the main thread. This includes reducing the complexity of tasks such as layout calculations, style recalculations, and reflows.
- Use Faster JavaScript Libraries:
- Choose lightweight and efficient JavaScript libraries that do not consume too much processing time and thus reduce the overall blocking time.
Tools to Measure TBT
There are several tools available that can help developers measure TBT and analyze page performance:
- Google Lighthouse: Provides insights into performance metrics, including TBT.
- Web Vitals Extension: A Chrome extension that provides real-time performance data, including TBT, directly on a website.
- Chrome DevTools: Can be used to analyze the main thread, JavaScript execution, and measure performance bottlenecks that contribute to TBT.
Conclusion
Total Blocking Time (TBT) is an important performance metric that reflects how quickly a page responds to user interactions, ultimately impacting user experience and engagement. While it is not a primary Core Web Vital, TBT plays a key role in influencing metrics like First Input Delay (FID) and user satisfaction. By optimizing JavaScript execution, reducing long tasks, and minimizing main-thread blocking, web developers can improve TBT and ensure a faster, more responsive experience for users.
As websites continue to compete for user attention, optimizing Core Web Vitals—including TBT—will become increasingly important for improving performance, search rankings, and, ultimately, business success.