Performance Profiling skill

Performance Profiling

Use this skill to diagnose Apple app performance issues systematically, pick the right profiling workflow, apply targeted fixes, and verify the change with real measurements.

Decision Tree

Choose the reference file before changing code:

What performance problem are you investigating?

+ App hangs, stutters, dropped frames, slow UI, high CPU
  -> Read references/time-profiler.md

+ High memory, leaks, OOM crashes, growing footprint
  -> Read references/memory-profiling.md

+ Slow cold launch, warm launch, resume, or time to first frame
  -> Read references/launch-optimization.md

+ Battery drain, thermal throttling, background energy, network waste
  -> Read references/energy-diagnostics.md

+ General "app feels slow"
  -> Start with references/time-profiler.md, then references/memory-profiling.md

+ Pre-release performance audit
  -> Read all reference files and use the review checklist below

Quick Reference

Problem	Instrument / Tool	Key Metric	Reference
UI hangs over 250 ms	Time Profiler + Hangs	Hang duration, main thread stack	references/time-profiler.md
High CPU usage	Time Profiler	CPU percent by function, call tree weight	references/time-profiler.md
Memory leak	Leaks + Memory Graph	Leaked bytes, retain cycle paths	references/memory-profiling.md
Memory growth	Allocations	Live bytes, generation analysis	references/memory-profiling.md
Slow launch	App Launch	Time to first frame, pre-main, post-main	references/launch-optimization.md
Battery drain	Energy Log	Energy impact, CPU/GPU/network activity	references/energy-diagnostics.md
Thermal issues	Activity Monitor, Instruments	Thermal state transitions	references/energy-diagnostics.md
Network waste	Network profiler	Redundant fetches, payload size	references/energy-diagnostics.md

Workflow

1. Identify the performance category from the user report, traces, logs, or code path.
2. Read only the matching reference file unless the issue is broad or unclear.
3. Prefer real device profiling with a Release build and representative data.
4. Inspect the code path named by the profile before proposing a fix.
5. Apply the smallest targeted fix that addresses the measured bottleneck.
6. Re-profile or add a repeatable measurement to confirm the improvement.

Profiling Ground Rules

• Profile on device when possible; Simulator uses host CPU and memory.
• Use Release configuration because optimizations can change hot paths.
• Reproduce with representative data, not empty databases or toy assets.
• Close unrelated apps to reduce noise during profiling.
• Keep measurements before and after the fix so the outcome is concrete.
• Add os_signpost markers when a workflow needs ongoing timing visibility.

Xcode Diagnostics

Recommend relevant Scheme > Run > Diagnostics settings when they match the suspected issue:

Setting	Use For
Main Thread Checker	UI work off the main thread
Thread Sanitizer	Data races and unsafe shared state
Address Sanitizer	Buffer overflows and use-after-free
Malloc Stack Logging	Allocation call stacks
Zombie Objects	Messages to deallocated objects

MetricKit Hook

Suggest MetricKit for production monitoring of launch, responsiveness, memory, and diagnostics:

import MetricKit

final class PerformanceReporter: NSObject, MXMetricManagerSubscriber {
    func startCollecting() {
        MXMetricManager.shared.add(self)
    }

    func didReceive(_ payloads: [MXMetricPayload]) {
        for payload in payloads {
            if let launch = payload.applicationLaunchMetrics {
                log("Resume time: \(launch.histogrammedResumeTime)")
            }
            if let responsiveness = payload.applicationResponsivenessMetrics {
                log("Hang time: \(responsiveness.histogrammedApplicationHangTime)")
            }
            if let memory = payload.memoryMetrics {
                log("Peak memory: \(memory.peakMemoryUsage)")
            }
        }
    }

    func didReceive(_ payloads: [MXDiagnosticPayload]) {
        for payload in payloads {
            if let hangs = payload.hangDiagnostics {
                for hang in hangs {
                    log("Hang: \(hang.callStackTree)")
                }
            }
        }
    }
}

Review Checklist

Responsiveness:

• No synchronous work on the main thread over 100 ms.
• No file I/O or network calls on the main thread.
• Large Core Data or SwiftData fetches use background contexts.
• Images decode off the main thread.
• @MainActor is limited to code that truly needs UI access.

Memory:

• No retain cycles in delegates, closures, observers, or async tasks.
• Large resources are released when no longer visible.
• Collections and caches are bounded.
• autoreleasepool is used in tight loops that create Objective-C objects.

Launch:

• No heavy work in init() of the @main App struct.
• Non-essential initialization is deferred.
• Dynamic frameworks are minimized where practical.
• No synchronous network calls occur during launch.

Energy:

• Background tasks use the appropriate BGTaskScheduler request type.
• Location accuracy matches the product need.
• Timers use tolerance so the system can coalesce wakeups.
• Network requests are batched and cached where possible.

References

• references/time-profiler.md: CPU profiling, hang detection, signpost API.
• references/memory-profiling.md: Allocations, Leaks, Memory Graph debugger.
• references/launch-optimization.md: Launch phases and cold/warm start optimization.
• references/energy-diagnostics.md: Battery, thermal state, and network efficiency.