What is the "High Total Blocking Time" check?

P2 · MediumPerformance

High Total Blocking Time

TBT exceeds 600ms, heavy main-thread blocking

Code: high_tbt

Why it matters

TBT exceeds 600ms, heavy main-thread blocking. Slow experiences increase bounce rates and hurt Core Web Vitals scores. Google uses these metrics as a direct ranking signal.

Related checks in Performance

Slow First Contentful Paint

FCP exceeds 3 seconds

FCP needs improvement

FCP between 1.8-3 seconds

Slow Largest Contentful Paint

LCP exceeds 4 seconds

LCP needs improvement

LCP between 2.5-4 seconds

Poor Cumulative Layout Shift

CLS exceeds 0.25

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