HTML / XML / JS, low-level validity (cross-cutting)MediumInferred
Render-blocking synchronous JS in head
Synchronous scripts in the head stop the browser parsing and painting until they load. Deferring or async-loading non-critical scripts lets the page render sooner.
What it is
Blocking scripts delay render.
Why it matters
Slows paint/LCP.
How to fix it
Defer/async non-critical JS.
How to find it on your site
- Run Lighthouse and read the render-blocking resources list.
- Find synchronous scripts in the head.
- Add defer or async, or move them, where safe.
- Re-measure first paint.
Cross-reference to ranking and citation factors
Render-blocking head scripts delay paint and LCP. Deferring them is a reliable speed win.
Impact
Low-medium. Inferred.
Evidence
Reduce render-blocking JS. web.dev, Core Web Vitals (Google); Google Search Central, Understanding page experience in Google Search results
Sources