When we talk about website performance, speed isn’t just about how fast a page loads. It’s also about how quickly users can start interacting with the page. One key metric that helps measure this is Total Blocking Time (TBT).
TBT measures the time a page is “blocked” from responding to user interactions, like clicking a button or typing in a form, while loading. In this article, we’ll explain what TBT is, why it matters, and how you can reduce it to create a faster, more responsive website.
Total Blocking Time (TBT) measures the amount of time a web page is blocked from responding to user interactions while it’s loading. Specifically, TBT looks at tasks that take longer than 50 milliseconds to execute. When a task takes too long, it blocks the main thread of the browser, meaning the page won’t respond to clicks, typing, or other interactions during that time.
TBT is measured from when the First Contentful Paint (FCP) occurs until the page is fully interactive. Lower TBT values mean users can interact with your page more quickly and smoothly.
- Good: 0-200 milliseconds: This is an ideal TBT (fast) and offers a highly responsive experience.
- Needs Improvement: 200-600 milliseconds: This range is generally acceptable but could benefit from optimization for a smoother experience.
- Poor: Over 600 milliseconds: This is considered slow, meaning users might experience delays when trying to interact with the page, which could impact their overall experience.
- Improved User Experience:
A low TBT means the page is responsive and ready for interaction sooner, which keeps users happy and engaged. Pages with high TBT can feel “frozen” or “unresponsive,” frustrating visitors.
- Better Core Web Vitals Score:
Google includes TBT as part of its Core Web Vitals, which are metrics used to evaluate user experience. Improving TBT can contribute to better SEO rankings, as search engines prioritize fast, user-friendly sites.
- Reduced Bounce Rate:
Visitors are more likely to leave a site that feels slow or unresponsive. By optimizing TBT, you can reduce bounce rates and increase the likelihood of conversions, sign-ups, or any other desired actions.
- Large JavaScript Files:
Heavy scripts take time to execute, which can block the main thread and make the page feel unresponsive.
- Long Tasks:
Any task that takes more than 50 milliseconds to complete is considered a “long task.” Many long tasks can add up, creating a high TBT.
- Heavy Client-Side Processing:
When a page relies heavily on JavaScript to render content, it can delay interactivity, increasing TBT.
- Synchronous JavaScript:
If JavaScript is loaded synchronously (in a way that blocks other tasks), it can hold up other parts of the page, delaying user interactions.
- Optimize and Minimize JavaScript:
Reduce the size of JavaScript files by minifying them. Break up large JavaScript files and load only what’s necessary on each page and use code splitting to divide JavaScript into smaller chunks.
- Defer or Async Non-Essential JavaScript:
Use the async or defer attribute for scripts that aren’t essential for initial content, so they don’t block the main thread. Also, prioritize loading critical scripts first and delay non-essential scripts until after the main content has loaded.
- Reduce Third-Party Scripts:
Third-party scripts, like ads, trackers, or social media widgets, can significantly increase TBT. Remove any unnecessary third-party scripts and consider alternatives that are lighter or more optimized.
- Break Up Long Tasks:
Use requestIdleCallback to schedule tasks during idle times, preventing them from blocking the main thread. Also, consider breaking down long-running tasks into smaller parts, allowing the browser to process them without blocking interactions.
- Optimize Web Workers:
Web Workers can run heavy JavaScript tasks in the background, away from the main thread. This allows the main thread to handle user interactions more effectively, reducing TBT.
- Use Lazy Loading for Images and Videos:
By loading images and videos only when they’re needed (i.e., when they enter the viewport), you reduce the load on the main thread, allowing the page to become interactive faster.
While TBT is essential for measuring interactivity, it’s helpful to evaluate it alongside First Input Delay (FID) and Largest Contentful Paint (LCP). FID measures the time it takes for the page to respond to the first user interaction, while LCP measures when the largest content element becomes visible. Together, these metrics provide a fuller picture of page performance and user experience.
Total Blocking Time (TBT) is a critical metric for ensuring your website feels smooth and responsive for visitors. By minimizing JavaScript, deferring non-essential scripts, and optimizing tasks, you can reduce TBT and create a more user-friendly experience. A lower TBT not only helps with SEO but also makes visitors more likely to stay, engage, and convert. Focusing on TBT as part of your performance optimization strategy will make your website faster, more responsive, and ultimately more successful.