Total Blocking Time (TBT)
Total Blocking Time measures the total time between First Contentful Paint and Time to Interactive where the main thread was blocked long enough to prevent input responsiveness. Tasks longer than 50ms contribute to TBT. It's a lab proxy for INP.
Why It Matters for Shopify Stores
TBT directly correlates with INP — the Core Web Vital measuring page responsiveness. A high TBT score means the main thread is occupied executing heavy JavaScript, making the page unresponsive to user interactions (taps, scrolls, clicks). For Shopify stores, high TBT is almost always caused by heavy app JavaScript executing on the main thread. Reducing TBT typically means reducing JavaScript load through fewer apps or deferred script loading.
How to Check Your Store
Google PageSpeed Insights reports TBT in the Performance Metrics section. Chrome DevTools > Performance tab shows main thread blocking in the task execution timeline.
Use the free shopify app bloat calculator toolHow to Fix It
Defer or remove heavy JavaScript. Break up long tasks. Use web workers for CPU-intensive operations. Reduce or replace apps with heavy JavaScript footprints.
Related Terms
INP (Interaction to Next Paint)
Core Web VitalsInteraction to Next Paint measures the responsiveness of your page to user interactions — clicks, taps, and keyboard input.
Time to Interactive (TTI)
Core Web VitalsTime to Interactive measures when a page becomes fully interactive — meaning the main thread is clear of long tasks and the page responds quickly to user input.
Render-Blocking Resources
PerformanceRender-blocking resources are CSS or JavaScript files that prevent a browser from displaying page content until they've fully downloaded and parsed.
JavaScript Defer / Async
PerformanceThe defer and async attributes on <script> tags change how JavaScript loads relative to HTML parsing.
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