Answer:

• Instruments — CPU Profiler, Time Profiler, Allocations, Leaks, Core Data
• Xcode Memory Graph — find retain cycles and leaked objects
• View Hierarchy Debugger — spot off-screen renders, overlapping views
• MetricKit — on-device production diagnostics
• os_signpost — custom time intervals in Instruments

Workflow for a slow scroll:

1. Profile with Instruments Time Profiler
2. Look for cellForRowAt taking too long
3. Move image decoding / layout off the main thread
4. Use os_signpost to confirm the improvement