My app feels fast on my development laptop. I have no idea how it performs on an average phone over a cellular connection, which is where a big share of my users will be. Set up realistic mobile performance testing: 1. Simulate the conditions: use dev tools device emulation with CPU throttling (mid-range mobile is roughly 4-6x slower than a dev laptop) and network throttling (a 'fast 3G' or 'slow 4G' profile). Walk through my core flows: landing page, signup, login, the main app screen, checkout if I have one. 2. For each flow, record: time until the page is usable (not just painted), scroll smoothness, tap responsiveness, and anything that janks, stutters, or blocks input. 3. Check the mobile-specific failure modes: layout that requires horizontal scrolling or pinch-zoom, tap targets too small or too close together, fixed headers eating half a small viewport, hover-dependent UI that doesn't work on touch, keyboards covering the input being typed into. 4. If possible, also test on at least one actual physical phone that isn't a flagship, on real cellular. Emulation misses things. 5. Produce a fix list for what you find, ranked by how badly it breaks the experience. Don't test only the landing page; the app behind the login is what paying users feel. The deliverable is the per-flow findings with measurements and the ranked fix list, plus the exact throttling settings used so I can reproduce.