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Metrics type: Supporting Metrics • Category: Website Performance

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At a glance

TTFB side-by-side mobile vs desktop, surfacing the device-specific server-response gap. Network-driven gap: mobile networks have higher latency than fixed broadband. Even the same edge POP serving the same content responds faster to broadband desktop users than 4G mobile users. Typical healthy gap: 200-500ms (desktop faster). Larger gaps signal mobile-network-specific issues or geographic distribution skew.

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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, TTFB per-template Wednesday 15 May 26. What the gap analysis is telling us:
  1. Site-wide TTFB gap of 460ms is in the typical band. Mobile users pay a 460ms penalty over desktop users for the same content; this is mostly mobile network RTT and is largely structural. Not a primary optimisation target, fixing TTFB overall (covered in psi_caching_opportunities) lifts both mobile and desktop together.
  2. Collection page gap of 640ms is borderline concerning. Could indicate the page has more cache-miss visitors on mobile (mobile users hitting different edge POPs) or some mobile-specific routing issue. Worth investigating CDN dashboard for cache hit rate per device.
  3. Cart and checkout gaps are very small because these pages are uncached (session-specific) on both devices; the structural CDN-cache difference disappears.
  4. The cumulative TTFB gap is mostly accepted as structural. Closing it from 460ms to 200ms requires aggressive mobile-edge-caching work that most CDNs handle automatically; further closing requires mobile-specific edge configurations rarely justified.
  5. The headline takeaway: mobile TTFB optimisation work is the same as overall TTFB optimisation work (CDN cache hit rate, origin response time). No mobile-specific TTFB fixes beyond ensuring CDN edge POPs are well-distributed near mobile user geographies.
The diagnostic flow:
  1. Confirm gap is in typical band. 200-500ms is acceptable.
  2. For concerning gaps, check CDN cache hit rate per device.
  3. Apply general TTFB fixes (cache configuration, origin optimisation), these benefit both devices.
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:
  • CDN dashboard, origin response time + cache hit rate per device.
  • CrUX BigQuery, TTFB per device.
Why the Vortex IQ TTFB gap may differ from CDN dashboard: Cross-connector reconciliation: primarily internal (with crux_ttfb_p75, psi_caching_opportunities). Quick rule for support tickets: TTFB gaps under 500ms are structural mobile-network differences. Larger gaps suggest CDN cache hit rate or routing issues per device.

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Known limitations / merchant FAQs

Why is mobile TTFB structurally slower? Mobile networks have higher latency (50-150ms) than fixed broadband. Even the same edge POP serving the same content responds slower to mobile clients. Most of the gap is structural and not directly fixable. Should I optimise mobile TTFB separately from desktop TTFB? No, generally. CDN caching and origin response improvements help both devices simultaneously. No mobile-specific TTFB fixes beyond ensuring CDN edge POPs are well-distributed near mobile user geographies. My mobile TTFB is 1,000ms higher than desktop. Is something broken? Possible. Investigate: (a) CDN cache hit rate per device in CDN dashboard; (b) whether mobile users hit different edge POPs than desktop; (c) whether origin has mobile-specific routing or response logic. Will HTTP/3 help mobile TTFB? Yes, modestly. HTTP/3 (QUIC) handles connection migration and packet loss better than HTTP/2, which benefits mobile users on lossy networks. Typical mobile TTFB improvement: 50-150ms.

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Tracked live in Vortex IQ Nerve Centre

Server Speed: Mobile vs Desktop 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.