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CLS Needs Improvement (0.1–0.25)

CLS between 0.1 and 0.25 is "needs improvement".

notice Impact: medium PERF_CLS_NEEDS_IMPROVEMENT 2 min read Updated

Why it matters

CLS between 0.1 and 0.25 is "needs improvement". Some layout shift detected but not catastrophic. Aim for < 0.1 to fully pass.

Address when convenient — notices usually mark a polish opportunity rather than a defect. Estimated SEO impact: medium — measurable effect on click-through or relevance.

How to fix

  • Same fixes as PERF_CLS_POOR — reserve space for images, ads, fonts

Common causes

If the rule is firing across many pages, the root cause is almost always one of these:

  • Render-blocking third-party scripts (analytics, chat, ads) loaded synchronously in <head>.
  • Hero images served at full original size with no responsive variants.
  • CSS bundle ships every component for every route instead of route-splitting.
  • A single uncached API call dominates time-to-interactive.

Anti-patterns to avoid

Even with the best intentions, these "fixes" make the issue worse — recognise them so you don't ship them:

  • Synchronous third-party scripts in <head>.
  • Serving 4K hero images on mobile because the desktop version "looked fine".
  • Disabling caching headers because "we want fresh content".

How atlookup detects this

Our crawler renders each page with a real headless browser, then collects Core Web Vitals (LCP, CLS, INP), payload sizes, and third-party request counts via Lighthouse. Pages where the rule fires for cls needs improvement (0.1–0.25) are flagged on the report.

If you'd like to see this rule fire on your own site, run a free 60-second audit — every page is reported with the exact lines that triggered it.

Tools to verify the fix

Once you've applied the fix, double-check with these external validators:

Frequently asked questions

Why does CLS Needs Improvement (0.1–0.25) matter for SEO?

CLS between 0.1 and 0.25 is "needs improvement". Some layout shift detected but not catastrophic. Aim for < 0.1 to fully pass.

How do I fix cls needs improvement (0.1–0.25)?

Same fixes as PERF_CLS_POOR — reserve space for images, ads, fonts

Is this a critical SEO issue?

Address when convenient — notices usually mark a polish opportunity rather than a defect. Estimated SEO impact: medium — measurable effect on click-through or relevance.

How does atlookup detect cls needs improvement (0.1–0.25)?

Our crawler renders each page with a real headless browser, then collects Core Web Vitals (LCP, CLS, INP), payload sizes, and third-party request counts via Lighthouse. Pages where the rule fires for cls needs improvement (0.1–0.25) are flagged on the report.

How long does it take to fix?

Under 5 minutes per page. Most teams batch similar issues across templates so the per-page time goes down at scale.

HTML_VERY_LARGE

HTML Document Extremely Large

Pages over several megabytes of HTML are often un-paginated lists or un-split SSR dumps.

PAGE_VERY_SLOW

Page Load Time Is Very Slow

Very slow pages (>5s load) fail Core Web Vitals thresholds and suffer dramatic CTR and conversion drop-offs.

PERF_CLS_POOR

Cumulative Layout Shift is Poor (> 0.25)

CLS measures unexpected layout shifts during page load.

PERF_TTFB_HIGH

High Time to First Byte

TTFB measures how long the server takes to respond.