Defer non-critical JavaScript loading to reduce main thread blocking.
- Defer non-critical JavaScript loading to reduce main thread blocking1 cited source1 category
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Why this matters
Delaying non-essential JS reduces input delay improving interaction responsiveness (INP).
Hypothetical standard operating procedure
Adapt this example to your site, access, tools, and change process. It is our suggested procedure, informed by the sources below.
- Identify non-critical JavaScript scripts in the page head or body.
- Add defer or async attributes to scripts to allow parallel parsing and delayed execution.
- Ensure essential bootstrap or hydration scripts load first but non-blocking.
- Use performance tools to verify reduction in long JS tasks and improved INP.
- Monitor user interaction delays in real user monitoring and lab tools.
Watch for
Render-blocking or synchronous JS can cause high input latency and INP scores.
Definition of done
Critical JS loads with defer or async attributes; page remains responsive under 200 ms INP.
Sources & attribution
Original checklist wording and example SOP by Best SEO Checklist, using source-grounded AI assistance. Editorial contact: Ted Kubaitis, @tedkubaitis on Teams. Check current source guidance before implementation.