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Metrics type: Supporting Metrics • Category: Website Performance

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At a glance

Lab Performance Score side-by-side mobile vs desktop, surfacing the gap between the two profiles. The “are we mobile-broken” diagnostic: mobile-first indexing means Google primarily ranks on mobile CWV, but most engineering teams develop on desktop and may miss mobile-specific issues. Typical healthy gap: 15-30 points (desktop higher). Larger gaps suggest mobile-specific problems (heavy mobile JS, mobile-only widgets, responsive image issues) that desktop development workflows don’t surface.

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Calculation

Calculated automatically from your Website Performance (PageSpeed + CrUX) data. See the At a glance summary above for what the metric tracks and the worked example below for a typical reading.

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Worked example

A UK-based BigCommerce fashion store, lab measurements per template Wednesday 15 May 26. What the comparison is telling us:
  1. Site-wide gap of 30 points sits at the borderline between “typical” and “concerning”. Desktop is healthy; mobile is dragging the overall picture. This is the textbook pattern for ecommerce sites built without dedicated mobile performance discipline.
  2. The homepage and collection pages have the largest gaps (34 and 31 points). Both pages have mobile-specific friction: the homepage hero carousel renders differently on mobile (4 unoptimised PNGs that are oversized for mobile viewport); the collection page’s filter widget runs heavy JS that mid-tier mobile CPUs struggle with.
  3. PDPs and checkout have typical gaps (24 and 21 points), suggesting mobile-specific work isn’t urgently needed there. The work to do concentrates on homepage + collection.
  4. The fix isn’t to “make desktop slower”, it’s to lift mobile to closer parity. Recommended optimisation focus:
    • Homepage: image responsive variants via srcset (saves 1.5-2MB on mobile); defer hero video carousel for mobile
    • Collection page: virtualise filter-widget grid on mobile; debounce filter clicks; defer non-critical JS
    • Estimated post-fix mobile score uplift: homepage 42 → 65, collection 48 → 70
  5. Why this matters for ranking: Google’s mobile-first indexing primarily uses mobile CWV. A 30-point mobile-vs-desktop gap signals mobile is the priority work; closing it lifts ranking impact.
  6. Compare to industry: brands that follow strict mobile-first development discipline operate at 5-15 point gaps. Brands like Shopify, Catalyst-on-BigCommerce, and modern Next.js sites often operate at <10 point gaps because their frameworks default to mobile-friendly patterns. Stencil-themed BC sites typically run 25-40 point gaps because Stencil defaults aren’t aggressively mobile-optimised.
The diagnostic flow:
  1. Read the gap; classify (0-15 healthy, 15-30 typical, 30-50 concerning, 50+ urgent).
  2. For concerning/urgent gaps, decompose by sub-metric: which CWV has the biggest mobile-vs-desktop gap?
  3. Apply mobile-specific fix patterns: responsive images, mobile-aware widget loading, mobile-specific JS deferral.
  4. Re-audit to confirm the gap closes.
Rapid-response playbook:

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Sibling cards merchants should reference together

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Reconciling against the vendor’s own dashboard

Where to look:
  • PageSpeed Insights, toggle between Mobile and Desktop tabs at the top of the report.
  • Chrome DevTools → Lighthouse panel, choose Mobile or Desktop device profile.
  • WebPageTest, supports custom device emulation including the same mobile/desktop split.
Why the Vortex IQ device gap may differ from PSI: Cross-connector reconciliation: primarily internal (with mobile/desktop pass rate cards and per-CWV device gap cards). Quick rule for support tickets: if a merchant says “my desktop is fast but mobile is slow, what should I do?”, the answer is in this card’s worked example, apply mobile-specific fix patterns (responsive images, mobile-aware widget loading, JS deferral on mobile).

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Known limitations / merchant FAQs

My desktop is 95 and mobile is 50. Where do I focus? Mobile, almost always. Mobile-first indexing means mobile is the priority for ranking. Desktop being healthy is good but doesn’t compensate for mobile being broken. Apply mobile-specific fix patterns: responsive images, mobile-aware JS, defer non-critical mobile widgets. Why is my mobile structurally slower than desktop? Three reasons: (1) Lighthouse mobile applies Slow 4G throttling (~5x slower than typical desktop wifi); (2) Lighthouse mobile applies 4x CPU slowdown; (3) mobile devices in real life have less compute power than desktop machines. Industry-typical: 15-30 point gap between mobile and desktop scores. Should I optimise to make mobile match desktop? Approximately, but not exactly. Mobile-target should be 70+ score (close to desktop’s 80-90 range, with the structural emulation gap accepted). Pushing past 80 mobile score requires architectural-level work (server-side rendering, edge functions, aggressive lazy-loading) that’s typically not justified by the marginal benefit. My BC Stencil site has a 35-point gap. Is that bad? Concerning band. Stencil sites without dedicated mobile optimisation typically run 25-40 point gaps. Catalyst (BC’s Next.js stack) typically runs 10-20 point gaps because it defaults to mobile-friendly patterns. Migration is one of the strongest mobile-performance wins alongside other Catalyst benefits. Can I have a smaller gap by making desktop slower? Theoretically yes; commercially absurd. The point of the gap analysis is to lift mobile, not to lower desktop. Don’t intentionally degrade desktop; protect it while improving mobile. Why does the gap matter if mobile is healthy at 70? It doesn’t, much. Once mobile is in healthy band (70+), the gap is informational rather than actionable. Concerning gaps are when mobile is in red band (under 50) while desktop is healthy, that’s the mobile-broken pattern.

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Tracked live in Vortex IQ Nerve Centre

Mobile vs Desktop Score is one of hundreds of KPI pulses Vortex IQ tracks across Website Performance (PageSpeed + CrUX) and 70+ other ecommerce connectors. Nerve Centre runs the detection layer; Vortex Mind investigates the cause when something moves; Ask Viq lets you interrogate any number in plain English. Start for free or book a demo to see this metric running on your own data.