Swift Performance Optimization

Language-level performance patterns for memory efficiency, runtime speed, and zero-cost abstractions. Profile first, optimize later — use this skill after Instruments identifies Swift code as the bottleneck.

When to Use

Use this skill when:

• Time Profiler shows Swift code as hotspot
• Excessive memory allocations or retain/release traffic
• Implementing performance-critical algorithms
• Writing framework code with performance requirements
• Optimizing tight loops or frequently called methods

Do NOT use for:

• First-step optimization (use performance-profiling first)
• SwiftUI performance (use swiftui-performance)
• Premature optimization

Example Prompts

• "Profiler shows excessive copying — how do I eliminate it?"
• "Retain/release overhead in Time Profiler — how to reduce?"
• "Generic code is slow in hot path"
• "Actor overhead causing UI jank"

What This Skill Provides

Core Topics

1. Noncopyable Types — Swift 6 ~Copyable for types that should never be copied
2. Copy-on-Write — Use isKnownUniquelyReferenced() and reserveCapacity()
3. Value vs Reference — Structs under 64 bytes are fast; larger need indirect storage
4. ARC Optimization — unowned is ~2x faster than weak
5. Generics — Use some over any for static dispatch
6. Collection Performance — ContiguousArray is ~15% faster than Array
7. Concurrency — Actor hops cost ~100μs; batch calls

Eliminate Copying

// ❌ Expensive copy
func process(_ data: ImageData) { ... }

// ✅ Zero-cost borrowing
func process(borrowing data: ImageData) { ... }

Actor Batching

// ❌ 10,000 actor hops (~1 second)
for _ in 0..<10000 {
    await counter.increment()
}

// ✅ Single hop (~0.1ms)
await counter.incrementBatch(10000)

Generic Specialization

// ❌ Dynamic dispatch
func render(shapes: [any Shape]) { ... }

// ✅ Static dispatch (10x faster)
func render<S: Shape>(shapes: [S]) { ... }

Code Review Checklist

• Large structs (>64 bytes) use indirect storage
• Collections use reserveCapacity when size known
• Prefer unowned over weak when lifetime guaranteed
• Use some instead of any where possible
• Batch actor calls in loops

Related

• performance-profiling — Use this first to identify bottlenecks
• swift-concurrency — Correctness-focused concurrency
• swiftui-performance — SwiftUI-specific optimization

Resources

WWDC: 2024-10229 (Swift performance), 2016-416 (Understanding Swift Performance)