Performance Profiling

Decision trees to choose the right Instruments tool and interpret results correctly. Covers Time Profiler, Allocations, Core Data, and Energy Impact.

When to Use This Skill

Use this skill when you're:

• App feels slow (UI lags, loads take 5+ seconds)
• Memory grows over time in Xcode memory gauge
• Battery drains fast (device gets hot)
• Unsure which Instruments tool to use
• Profiling results are confusing or contradictory
• Want to profile proactively before users complain

Core principle: Measure before optimizing. Guessing wastes more time than profiling.

Example Prompts

Questions you can ask Claude that will draw from this skill:

• "My app feels slow. Which Instruments tool should I use?"
• "How do I read Time Profiler results? What's the difference between Self Time and Total Time?"
• "Memory keeps growing but I don't see leaks. What's happening?"
• "How do I find N+1 queries with Core Data instrument?"
• "My app drains battery. How do I profile energy usage?"
• "Time Profiler shows a hot spot but fixing it didn't help. What did I miss?"
• "How do I profile a specific user interaction?"

What's Covered

Decision Tree

Time Profiler Deep Dive

• Self Time vs Total Time (critical distinction)
• Call tree navigation
• Identifying hot spots
• Common misinterpretations

Allocations Deep Dive

• Object creation tracking
• Growth patterns (leak vs cache)
• Heap snapshots
• Mark Generation technique

Core Data Deep Dive

• N+1 query detection with SQL logging
• Prefetching patterns
• Batch operation optimization

Energy Impact

• CPU/GPU/Network subsystem analysis
• Background activity auditing
• Location services impact

Pressure Scenarios

• Deadline pressure handling
• Manager authority pressure
• Misinterpretation prevention

Key Pattern

Self Time vs Total Time

Self Time: Time spent IN this function (not callees)
Total Time: Time spent in this function AND all callees

Example reading:
┌─────────────────────────────┬──────────┬────────────┐
│ Function                    │ Self Time│ Total Time │
├─────────────────────────────┼──────────┼────────────┤
│ processData()               │ 2ms      │ 500ms      │ ← NOT the problem
│   └─ expensiveSort()        │ 498ms    │ 498ms      │ ← THE problem
└─────────────────────────────┴──────────┴────────────┘

✅ Look at Self Time to find actual bottleneck
❌ Don't optimize processData() based on Total Time alone

Quick Profiling Workflow

1. Profile on real device (not simulator)
2. Record specific interaction (not just app launch)
3. Look at Self Time first
4. Find 80/20 (20% of code causing 80% of time)
5. Measure after fix (verify improvement)

Documentation Scope

This page documents the axiom-performance-profiling skill—Instruments decision trees and workflows Claude uses when helping you profile performance issues. The skill contains complete tool tutorials, real-world examples, and pressure scenario handling.

For SwiftUI-specific profiling: See swiftui-performance for the new SwiftUI Instrument in Instruments 26.

For memory leaks specifically: See memory-debugging for retain cycle diagnosis.

Related

• swiftui-performance — SwiftUI Instrument for view update analysis
• memory-debugging — Retain cycle and leak diagnosis
• xcode-debugging — Environment-first diagnostics before profiling
• swift-concurrency — Background thread patterns for CPU work

Resources

WWDC: 2023-10160, 2024-10159, 2025-239

Docs: /instruments, /xcode/performance