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Nexora Web Design
Performance

What is TBT (Total Blocking Time)?

Lab-data metric that sums all the time JavaScript blocked the main thread between First Contentful Paint and Time to Interactive.

Also known as: Total Blocking Time, main-thread blocking

Total Blocking Time is a lab metric (measured by Lighthouse) that adds up every chunk of main-thread blocking longer than 50ms during page load. It's the lab-data proxy for what INP measures in the field.

A good TBT is under 200ms. Between 200-600ms is 'needs improvement.' Over 600ms is poor.

Unlike INP (which Google now uses for ranking), TBT isn't a Core Web Vital directly — but it's the metric you'll see flagged in Lighthouse reports, PageSpeed Insights, and most performance dashboards. Fixing TBT and fixing INP overlap heavily: defer non-critical JS, code-split large bundles, lazy-load third-party widgets.

Why it matters

TBT is the most actionable performance metric for developers — it shows up clearly in Lighthouse with which scripts are causing the blocking.

Related

More Performance terms

INP (Interaction to Next Paint)

How long the page takes to visually respond to a user interaction (tap, click, keypress). Replaced FID as the official Core Web Vitals responsiveness metric in March 2024.

FID (First Input Delay)

Deprecated Core Web Vital that measured only the first user interaction. Replaced by INP in March 2024.

Core Web Vitals

Google's three measurements of page experience — loading speed (LCP), interactivity (INP), and visual stability (CLS). A real ranking signal since 2024.

LCP (Largest Contentful Paint)

How long it takes for the largest visible element (hero image, headline, etc.) to render. Google's primary loading-speed metric.

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