> ## Documentation Index > Fetch the complete documentation index at: https://docs.vortexiq.ai/llms.txt > Use this file to discover all available pages before exploring further. # Server Speed: Mobile vs Desktop, Website Performance (PageSpeed + CrUX) > Server Speed: Mobile vs Desktop for Website Performance (PageSpeed + CrUX) stores. Tracked live in Vortex IQ Nerve Centre. How to read it, why it matters, ... **Metrics type:** [Supporting Metrics](/nerve-centre/overview#metrics-types-explained) • **Category:** [Website Performance](/nerve-centre/connectors#connectors-by-type) ## 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. | | | | ------------------------------------------ | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | **What it counts** | TTFB measurements from CrUX field data, mobile vs desktop. Reported side-by-side with the gap. | | **Sample type** | **Field data** sourced from CrUX. Lab equivalent (per-device): [`psi_server_response`](/nerve-centre/kpi-cards/website-performance/server-response-time). | | **Why mobile TTFB is structurally slower** | (1) **Mobile network latency**: 4G/5G mobile typically adds 50-150ms to RTT vs broadband. (2) **Mobile users in worse network conditions**: signal strength variability, network congestion, edge-of-coverage. (3) **Mobile geographic mix**: mobile users are more frequently on the move and may hit different edge POPs than desktop users from fixed locations. | | **Healthy gap interpretation** | **0-200ms gap**: server is fast enough that mobile network latency is the only structural difference. **200-500ms gap**: typical. **500-1,000ms gap**: meaningful mobile network or routing issues. **1,000+ms gap**: indicates major mobile-vs-desktop infrastructure difference (CDN POPs not optimised for mobile RTT, origin response time variable). | | **Reading the gap diagnostically** | TTFB gap is mostly determined by network and CDN factors, not by site code changes. **Common causes of large gaps**: (a) CDN POPs serving mobile traffic from distant edges; (b) mobile users on poor-quality networks (some carriers have higher RTT); (c) CDN cache-miss rate higher for mobile (different caching keys, mobile-specific URLs). | | **Currency** | n/a, two milliseconds + delta. | | **Time window** | `28D rolling` (CrUX field) per device profile. | | **Alert trigger** | `gap > 800ms` (mobile-network or routing issue). | | **Sentiment key** | `psi_ttfb` | | **Roles** | owner, operations | ## 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. ## Worked example A UK-based BigCommerce fashion store, TTFB per-template Wednesday 15 May 26. | Page template | Mobile TTFB | Desktop TTFB | Gap | Read | | ------------------- | ----------: | -----------: | --------: | --------------- | | Homepage | 1,180ms | 720ms | 460ms | Typical | | Product detail page | 1,420ms | 880ms | 540ms | Typical | | Collection page | 1,680ms | 1,040ms | **640ms** | Borderline | | Cart | 380ms | 240ms | 140ms | Healthy | | Checkout | 540ms | 320ms | 220ms | Typical | | **Site p75** | **1,180ms** | **720ms** | **460ms** | **Typical gap** | 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`](/nerve-centre/kpi-cards/website-performance/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: | Time horizon | Action | | -------------- | ---------------------------------------------------------- | | First 1 hour | Confirm gap classification. | | First 24 hours | If concerning, investigate CDN cache hit rate per device. | | Day 28 | Field TTFB per device reflects any infrastructure changes. | ## Sibling cards merchants should reference together | Card | Why merchants reach for it | | ---------------------------------------------------------------------------------------------------------- | ------------------------------------ | | [`psi_mobile_vs_desktop_score`](/nerve-centre/kpi-cards/website-performance/mobile-vs-desktop-score) | Composite score gap. | | [`psi_mobile_desktop_lcp`](/nerve-centre/kpi-cards/website-performance/load-speed-mobile-vs-desktop) | LCP gap. | | [`psi_mobile_desktop_inp`](/nerve-centre/kpi-cards/website-performance/responsiveness-mobile-vs-desktop) | INP gap. | | [`psi_mobile_desktop_cls`](/nerve-centre/kpi-cards/website-performance/layout-stability-mobile-vs-desktop) | CLS gap. | | [`crux_ttfb_p75`](/nerve-centre/kpi-cards/website-performance/server-response-time) | Field TTFB. | | [`psi_server_response`](/nerve-centre/kpi-cards/website-performance/server-response-time) | Lab TTFB. | | [`psi_caching_opportunities`](/nerve-centre/kpi-cards/website-performance/caching-opportunities) | CDN caching, the primary TTFB lever. | ## 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:** | Reason | Direction | What to do | | ----------------------------------------------------------------------------------------------------- | ------------------ | --------------------------------------- | | **TTFB definition.** CrUX includes DNS + TCP + TLS + server time; CDN dashboards measure server-only. | CrUX **higher** | Use CrUX for user-perception decisions. | | **Window timing.** | Vortex IQ **lags** | Wait for refresh. | **Cross-connector reconciliation:** primarily internal (with [`crux_ttfb_p75`](/nerve-centre/kpi-cards/website-performance/server-response-time), [`psi_caching_opportunities`](/nerve-centre/kpi-cards/website-performance/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. ## 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. *** ### 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](https://app.vortexiq.ai/login) or [book a demo](https://www.vortexiq.ai/contact-us) to see this metric running on your own data.