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How to reduce Total Blocking Time (TBT)

Speed1 min readFixes check tbt

TBT adds up every moment the page is frozen by JavaScript while loading. Under 200 ms keeps the page responsive. It is the lab stand-in for INP.

Step by step, with screenshots: Core Web Vitals for site owners — LCP, INP and CLS without jargon →

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What a passing site looks like

  • Total Blocking Time: passesfail · −30 ptseffort L

Total Blocking Time

Why it matters. TBT adds up every moment the page was too busy running JavaScript to respond to a tap. It is the lab stand-in for INP and the heaviest-weighted Lighthouse metric.

How to fix it.
  1. Remove or defer JavaScript that is not needed for the first screen (defer, async, or load on interaction).
  2. Split long tasks (> 50 ms) into smaller chunks; move heavy work to a web worker.
  3. Audit third-party tags — chat widgets, tag managers and A/B tools are the usual culprits.

Filed under Speed & Core Web Vitals. Copy is generated from the same catalogue that scores every report, so what you read here is what the report says.