A lightweight site should deliver useful server-rendered content and core actions with small assets, minimal script dependence, and progressive enhancement so it remains usable on slow networks and modest devices.
Compare network conditions, viewport behavior, asset delivery, script cost, responsive images, menus, fonts, and forms to isolate problems that appear mainly on phones.
Measure what mobile visitors download and wait for, then target the largest delays in server response, images, scripts, fonts, rendering, and third-party resources.