Performance fix guide
Long Main Thread Tasks Detected
JavaScript tasks exceeding 50ms block the main thread, causing jank and poor responsiveness. This directly impacts INP and user experience.
Issue ID: CWV-LONG-TASKS-001
Severity: major
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
JavaScript tasks exceeding 50ms block the main thread, causing jank and poor responsiveness. This directly impacts INP and user experience.
Why it matters
JavaScript tasks exceeding 50ms block the main thread, causing jank and poor responsiveness. This directly impacts INP and user experience. 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 CWV-LONG-TASKS-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 long tasks using requestIdleCallback or setTimeout(0)
- 2Move heavy computation to Web Workers
- 3Defer non-critical JavaScript execution
How to re-check it
- Use Chrome DevTools Performance panel to confirm no tasks exceed 50ms
Related tools
This issue is best verified with the full FreeSiteAudit crawl rather than a single-point mini tool.