Total Blocking Time (TBT)
Total Blocking Time (TBT) measures how long the browser’s main thread is blocked by long tasks during page loading, limiting its ability to respond to user input.
What is Total Blocking Time?
Total Blocking Time (TBT) is a lab performance metric that measures how long a page’s main thread is blocked by long tasks during loading. When the main thread is busy, the browser cannot respond quickly to clicks, taps or keyboard input. TBT is commonly surfaced in Google Lighthouse and PageSpeed Insights.
How is Total Blocking Time calculated?
A long task is a main-thread task that runs for more than 50 milliseconds. TBT adds up the blocking portion of each long task: the time beyond that 50 ms threshold. A 120 ms task therefore contributes 70 ms to TBT.
Unlike Core Web Vitals, TBT is a lab metric rather than a field metric based on real-user data. It is useful for diagnosing responsiveness problems before or after publishing website changes.
What causes high TBT?
Heavy JavaScript is a common cause. Large scripts, third-party tools, tag managers and complex interactions can occupy the main thread for too long. On a Webflow website, marketers should pay particular attention to custom scripts and third-party integrations added over time.
Why does TBT matter for B2B websites?
B2B pages often combine analytics, CRM scripts, chat tools, video embeds and interactive components. Each addition can increase processing work. A slow landing page can feel unresponsive even when its visual content appears quickly, creating unnecessary friction before a form submission or call to action.
How do you improve Total Blocking Time?
Start by running a performance audit and identifying the longest main-thread tasks. Remove unnecessary JavaScript, delay non-critical third-party scripts, reduce script size and break expensive work into smaller tasks. Treat TBT as a diagnostic metric within broader Webflow SEO and performance work, not as an isolated score to maximize.
Key takeaway
Total Blocking Time helps identify code that prevents a page from responding quickly during loading. For B2B marketers, it is most useful for finding performance costs introduced by scripts, integrations and interactive functionality before they undermine the experience of a Webflow marketing website.
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