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.
nimo audit lens
How nimo reads TBT in an audit.
The glossary explains the metric. The free audit checks the page, separates field data from lab diagnostics, and points at the first review step.
How nimo interprets it
nimo uses TBT as a lab signal for main-thread pressure and INP risk, not as a real-user metric.
Source to trust first
Use Lighthouse TBT for immediate debugging. Trust CrUX INP when deciding whether visitors feel a responsiveness problem.
Evidence to inspect
- Long tasks during startup and hydration.
- Script evaluation and execution cost.
- Third-party tag and tag-manager work.
- Unused or page-wide JavaScript loaded before interaction.
First fix to review
Split or defer the script work that blocks the main thread during the first load.
How to validate
Rerun Lighthouse and compare TBT and long-task count, then use INP field data to decide whether more responsiveness work is needed.
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
Run the free audit, then ask: “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.