Performance metric
What is TBT?
Total Blocking Time is a Lighthouse lab metric that estimates how long JavaScript blocks the browser from responding during page load. It is not a field metric, but it is useful for diagnosing INP risk.
Plain-English version
How long scripts freeze the browser (lab only).
Target
Under 200ms
TBT thresholds
Use thresholds as a triage tool. Field data matters most when there is enough real Chrome traffic for the page.
Good
≤ 200ms
Needs work
200–600ms
Poor
> 600ms
Why it matters
What a visitor feels when TBT is bad.
Why TBT matters
High TBT often explains why a page feels busy or delayed before it becomes usable. It is a strong clue that JavaScript is competing with user interactions.
What a poor result usually means
A poor TBT usually means large bundles, expensive hydration, third-party scripts, tag managers, or long tasks are occupying the main thread.
What to fix first
Do not chase the score. Fix the bottleneck.
The right fix depends on the metric, the page template, and whether the issue appears in real visitor data.
- Split large JavaScript bundles and load only what the page needs.
- Defer non-critical scripts and audit tag manager containers.
- Reduce hydration work on pages with heavy interactive components.
- Break long tasks into smaller units.
- Remove third-party scripts that do not justify their cost.
How nimo helps
Ask nimo: “What scripts are blocking my main thread?”
nimo treats TBT as a diagnostic clue, then checks whether real visitor responsiveness data supports the same concern.
No signup needed. Takes about 30 seconds.