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. |
| 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 |
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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. - 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.
- Cart and checkout gaps are very small because these pages are uncached (session-specific) on both devices; the structural CDN-cache difference disappears.
- 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.
- 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.
- Confirm gap is in typical band. 200-500ms is acceptable.
- For concerning gaps, check CDN cache hit rate per device.
- Apply general TTFB fixes (cache configuration, origin optimisation), these benefit both devices.
| 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 | Composite score gap. |
psi_mobile_desktop_lcp | LCP gap. |
psi_mobile_desktop_inp | INP gap. |
psi_mobile_desktop_cls | CLS gap. |
crux_ttfb_p75 | Field TTFB. |
psi_server_response | Lab TTFB. |
psi_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.
| 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. |
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.