Web Engineering11 May 2026 · 5 min read
The Core Web Vitals that actually move conversion on Indian networks
LCP under 2.5s is the textbook number. On a Jio 4G connection in a Tier-2 city, the real threshold is closer to 1.9s. Here is why.
Karthik R.
Principal Engineer, Ampex Web
Google's thresholds are global averages. For a checkout page served to a mid-range Android phone on a congested 4G tower in Nagpur, those averages hide a factor-of-two variance.
What we measure instead
We track P75 LCP segmented by device tier and connection type, using field data from real users — not lab runs. The gap between a Pixel 8 on WiFi and a Redmi 12 on 4G is often 4–5×.
- Ship a critical-CSS budget under 14KB so the first paint fits in one TCP round-trip
- Preload the hero image with fetchpriority=high; skip lazy-loading above the fold
- Defer every third-party script that is not required for the primary CTA
The conversion delta
On the D2C accounts we run, pulling P75 LCP from 3.1s to 1.9s lifted mobile checkout completion by 11–18%. That is the whole business case for a performance sprint.