Diagnose a Performance Problem

Background: Native [iOS / Android] app, [SwiftUI / Compose]. The symptom:
[describe—jank, slow screen, hangs, high memory, battery drain, ANR].
When it happens: [which screen/action]. What I've observed so far:
[any measurements or "none yet"].

Use case: Give me a methodical plan to find the actual cause with the
profiler—before I change any code.

Implementation details:
- iOS: which Instruments template fits this symptom (Time Profiler,
  Allocations/Leaks, SwiftUI, Animation Hitches, Energy Log) and what to
  look for. Android: which Android Studio profiler / tool (CPU, Memory,
  Energy, the System Trace / Perfetto, layout inspector) and what to look
  for.
- A hypothesis-driven loop: measure → form hypothesis → isolate → verify
- How to reproduce reliably and measure on a real low-end device, not
  just the simulator/emulator or a flagship
- Quantify before/after so I know a change actually helped

Deliverable: A step-by-step diagnosis plan for this symptom, the specific
profiler tool and what metric to read, the reproduction/measurement
setup, and how to confirm a fix with numbers.