Total Blocking Time (TBT): Crawl Audit check 76

Short answer: Total Blocking Time (TBT) adds up how long the browser was too busy running JavaScript to respond, while the page loaded. It's a lab metric, measured by Lighthouse rather than real visitors, and it tends to move with INP (check 73). It carries the biggest weight in Lighthouse's performance score. Check 76 passes at 200ms or less, warns up to 600ms and fails above that.

What Crawl Audit checks

• Full report: Lighthouse's TBT from PageSpeed Insights. There's no real-user version of this metric.
• Free check: an estimate of 45ms per external script on the page. A rough signal, not a measurement.
• Pass: 200ms or less. Warning: up to 600ms. Fail: over 600ms. These match Lighthouse's mobile bands.

What Google says

• Lighthouse: "Any task that executes for more than 50 ms is a long task." TBT counts the part of each long task over 50ms.
• web.dev: "A low TBT often correlates with a low Interaction to Next Paint (INP)." But "TBT may be a reasonable proxy metric for INP for the lab but it's not a substitute for INP in and of itself."
• In Lighthouse's performance score, TBT counts for 30%, the most of any metric (LCP and CLS 25% each, FCP and Speed Index 10% each).

On devmarde.com

On 3 October, PageSpeed Insights measured our homepage TBT at 30ms on mobile. It still listed about 74 KiB of unused JavaScript (check 85), so there's room to trim.

How to cut it

• Remove scripts and plugins you don't use.
• Load third-party scripts with defer, or after the page is ready (check 99).
• Split large bundles so each page only loads the code it needs.

Sources

• Chrome for Developers, Total Blocking Time and Lighthouse performance scoring, checked 3 October 2026.
• web.dev, Total Blocking Time (TBT) (updated 15 October 2025).