performance-profiling

First step: Tell the user: "performance-profiling skill loaded."

Performance Profiling

Systematic guide for profiling Apple platform apps using Instruments, Xcode diagnostics, and MetricKit. Covers CPU, memory, launch time, and energy analysis with actionable fix patterns.

When This Skill Activates

Use this skill when the user:

• Reports app hangs, stutters, or dropped frames
• Needs to profile CPU usage or find hot code paths
• Has memory leaks, high memory usage, or OOM crashes
• Wants to optimize app launch time
• Needs to reduce battery/energy impact
• Asks about Instruments, Time Profiler, Allocations, or Leaks
• Wants to add os_signpost or performance measurement to code
• Is preparing for App Store review and needs performance validation

Decision Tree

What performance problem are you investigating?
│
├─ App hangs / stutters / dropped frames / slow UI
│  └─ Read references/time-profiler.md
│
├─ High memory / leaks / OOM crashes / growing footprint
│  └─ Read references/memory-profiling.md
│
├─ Slow app launch / time to first frame
│  └─ Read references/launch-optimization.md
│
├─ Battery drain / thermal throttling / background energy
│  └─ Read references/energy-diagnostics.md
│
├─ General "app feels slow" (unknown cause)
│  └─ Start with references/time-profiler.md, then references/memory-profiling.md
│
└─ Pre-release performance audit
   └─ Read ALL reference files, use Review Checklist below

Quick Reference

Problem	Instrument / Tool	Key Metric	Reference
UI hangs > 250ms	Time Profiler + Hangs	Hang duration, main thread stack	references/time-profiler.md
High CPU usage	Time Profiler	CPU % 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 score, CPU/GPU/network	references/energy-diagnostics.md
Thermal issues	Activity Monitor	Thermal state transitions	references/energy-diagnostics.md
Network waste	Network profiler	Redundant fetches, large payloads	references/energy-diagnostics.md

Process

1. Identify the Problem Category

Ask the user or inspect their description to classify the issue:

• Responsiveness: Hangs, stutters, animation drops
• Memory: Leaks, growth, OOM crashes
• Launch: Slow cold/warm start
• Energy: Battery drain, thermal throttling

2. Read the Appropriate Reference File

Each file contains:

• Which Instruments template to use
• Step-by-step profiling workflow
• How to interpret results
• Common fix patterns with code examples

3. Profile on Real Hardware

Always remind users:

• Profile on device, not Simulator (Simulator uses host CPU/memory)
• Use Release build configuration (optimizations change behavior)
• Profile with representative data (empty databases hide real perf)
• Close other apps to reduce noise

4. Apply Fixes and Verify

After identifying bottlenecks:

• Apply targeted fix from the reference file
• Re-profile to confirm improvement
• Add os_signpost markers for ongoing monitoring

Xcode Diagnostic Settings

Recommend enabling these in Scheme > Run > Diagnostics:

Setting	What It Catches
Main Thread Checker	UI work off main thread
Thread Sanitizer	Data races
Address Sanitizer	Buffer overflows, use-after-free
Malloc Stack Logging	Memory allocation call stacks
Zombie Objects	Messages to deallocated objects

MetricKit Integration

For production monitoring, recommend MetricKit:

import MetricKit

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

    func didReceive(_ payloads: [MXMetricPayload]) {
        for payload in payloads {
            // Launch time
            if let launch = payload.applicationLaunchMetrics {
                log("Resume time: \(launch.histogrammedResumeTime)")
            }
            // Hang rate
            if let responsiveness = payload.applicationResponsivenessMetrics {
                log("Hang time: \(responsiveness.histogrammedApplicationHangTime)")
            }
            // Memory
            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 main thread > 100ms
• No file I/O or network calls on main thread
• Core Data / SwiftData fetches use background contexts for large queries
• Images decoded off main thread (use .preparingThumbnail or async decoding)
• @MainActor only on code that truly needs UI access

Memory

• No retain cycles (check delegate patterns, closures with self)
• Large resources freed when not visible (images, caches)
• Collections don't grow unbounded (capped caches, pagination)
• autoreleasepool used in tight loops creating ObjC objects

Launch Time

• No heavy work in init() of @main App struct
• Deferred non-essential initialization (analytics, prefetch)
• Minimal dynamic frameworks (prefer static linking)
• No synchronous network calls at launch

Energy

• Background tasks use BGProcessingTaskRequest appropriately
• Location accuracy matches actual need (not always .best)
• Timers use tolerance to allow coalescing
• Network requests batched 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 — App launch phases, cold/warm start optimization
• references/energy-diagnostics.md — Battery, thermal state, network efficiency
• WWDC: Ultimate Application Performance Survival Guide
• WWDC: Analyze Hangs with Instruments
• WWDC: Detect and Diagnose Memory Issues