G24 — Chapter 7 - Technical SEO

Criterion G24 : Defer/Async JS — guide + checklist

PART 1 - Fundamentals Chapter 7 - Technical SEO Keyword : defer/async js

This criterion is subtle… until it blocks SEO performance.

The criterion **G24 — Defer/Async JS** is part of our SEO checklist (335 criteria). Here, you have a **practical** method to check and fix it — with a concrete example.

What exactly this criterion covers

This criterion is subtle… until it blocks SEO performance.

**G24 — Defer/Async JS** (Chapter 7 - Technical SEO): defer for non-critical JS, async for analytics, avoid render-blocking

Why it matters (SEO + UX)

Why it matters: it is a technical quality factor (crawl, rendering, indexing). When poorly applied, we often observe: ambiguity (wrong associated query), duplication between pages, or loss of performance on impressions.

On high-volume generated sites, this criterion also serves as a **safeguard**: a stable rule prevents 1,000 errors at once.

How to check (step by step)

Approach: browser-side control (render + code). Recommended tool: **Chrome DevTools Network**.

  1. Open the page in Chrome → DevTools → Performance/Network tab.
  2. Run Lighthouse and note the main weakness.
  3. Check if the issue repeats on templates.

Tip: first isolate 10 “representative” URLs (top pages + generated pages) before scaling the fix.

How to fix properly

Strategy: apply a rule, then check neighboring pages.

  • Fix the biggest cost source (images, JS, fonts, cache).
  • Retest, then apply to the template (not page by page).
  • Add a safeguard: weight budget (KB) and CI check if possible.

Then: re-crawl 50–200 URLs, then monitor Search Console for 7–14 days (impressions/CTR/indexing).

Concrete example (illustrative)

Example (illustrative):

  • **Context**: blog article for law firm in Paris
  • **Before**: Lighthouse: 52/100 (heavy JS, unoptimized images).
  • **After**: Lighthouse: 93/100 (lazy-load, compression, cache).
  • **Note**: Goal: stabilize CLS.

Checklist to tick

  • [ ] Measure before/after
  • [ ] Respects: avoid render-blocking
  • [ ] Improvement on template
  • [ ] No CWV regression
  • [ ] Cache and compression OK
FAQ

Frequently asked questions — G24

What is the most common mistake on “Defer/Async JS”?

Fixing an isolated page without fixing the template/import: the error returns on the next generation.

Which tool is fastest for large-scale checking?

For this type of criterion, a crawl (e.g. Screaming Frog) + targeted verification in Chrome DevTools Network is generally the fastest combo.

How to prevent this from recurring on 10K generated pages?

Freeze an auto-generation rule (title/structure/schema/URLs) + add automatic control (crawl or test) before production import.

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