Performance fix guide
Render-Blocking JavaScript Detected
Synchronous <script> tags in the <head> block page rendering. The browser must download and execute these scripts before painting any content.
Issue ID: CWV-RENDER-BLOCK-JS-001
Severity: moderate
Impact: Med
Effort: M
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
Synchronous <script> tags in the <head> block page rendering. The browser must download and execute these scripts before painting any content.
Why it matters
Synchronous <script> tags in the <head> block page rendering. The browser must download and execute these scripts before painting any content. 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-RENDER-BLOCK-JS-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
- 1Add defer or async attributes to non-critical scripts
- 2Move scripts to the bottom of <body>
- 3Use dynamic import() for non-essential JavaScript
How to re-check it
- Run Lighthouse and confirm no render-blocking JS warnings
Related tools
This issue is best verified with the full FreeSiteAudit crawl rather than a single-point mini tool.