Performance fix guide
High Total Blocking Time (TBT)
Page is unresponsive during loading due to heavy JavaScript.
Issue ID: PERF-TBT-001
Severity: high
Impact: High
Effort: L
Use this article when
- You need deeper remediation guidance than the issue card can show.
- You want CMS-specific steps before handing the fix to a developer.
- You want a repeatable re-check path after shipping the change.
What this issue is
Page is unresponsive during loading due to heavy JavaScript.
Why it matters
Page is unresponsive during loading due to heavy JavaScript. This affects load speed, Core Web Vitals, and how many visitors stay long enough to convert.
How we detect it
- FreeSiteAudit flags this issue when the rule for PERF-TBT-001 fails and the page evidence points to Performance tool.
- You can usually confirm this by checking the page source or the relevant page settings inside your CMS.
Evidence examples
Check the affected page source, rendered output, or relevant CMS setting to confirm the missing or incorrect element.
How to fix it
- 1Break up long JavaScript tasks
- 2Defer non-critical JS
- 3Remove unused JavaScript
- 4Use web workers for heavy computation
How to re-check it
- Run Lighthouse and confirm TBT < 300ms