Performance Optimization — Expert Decisions
Expert decision frameworks for performance choices. Claude knows lazy loading and async basics — this skill provides judgment calls for when to optimize and which tool to use.
Decision Trees
Should You Optimize?
When should you invest in optimization?
├─ User-facing latency issue (visible stutter/delay)
│ └─ YES — Profile and fix
│ Measure first, optimize second
│
├─ Premature concern ("this might be slow")
│ └─ NO — Wait for evidence
│ Write clean code, profile later
│
├─ Battery drain complaints
│ └─ YES — Use Energy Diagnostics
│ Focus on background work, location, network
│
├─ Memory warnings / crashes
│ └─ YES — Use Allocations + Leaks
│ Find retain cycles, unbounded caches
│
└─ App store reviews mention slowness
└─ YES — Profile real scenarios
User perception matters
The trap: Optimizing based on assumptions. Always profile first. The bottleneck is rarely where you think.
Profiling Tool Selection
What are you measuring?
├─ Slow UI / frame drops
│ └─ Time Profiler + View Debugger
│ Find expensive work on main thread
│
├─ Memory growth / leaks
│ └─ Allocations + Leaks instruments
│ Track object lifetimes, find cycles
│
├─ Network performance
│ └─ Network instrument + Charles/Proxyman
│ Latency, payload size, request count
│
├─ Disk I/O issues
│ └─ File Activity instrument
│ Excessive reads/writes
│
├─ Battery drain
│ └─ Energy Log instrument
│ CPU wake, location, networking
│
└─ GPU / rendering
└─ Core Animation instrument
Offscreen rendering, overdraw
SwiftUI View Update Strategy
View is re-rendering too often?
├─ Caused by parent state changes
│ └─ Extract to separate view
│ Child doesn't depend on changing state
│
├─ Complex computed body
│ └─ Cache expensive computations
│ Use ViewModel or memoization
│
├─ List items all updating
│ └─ Check view identity
│ Use stable IDs, not indices
│
├─ Observable causing cascading updates
│ └─ Split into multiple @Published
│ Or use computed properties
│
└─ Animation causing constant redraws
└─ Use drawingGroup() or limit scope
Rasterize stable content
Memory Management Decision
How to fix memory issues?
├─ Steady growth during use
│ └─ Check caches and collections
│ Add eviction, use NSCache
│
├─ Growth tied to navigation
│ └─ Check retain cycles
│ weak self in closures, delegates
│
├─ Large spikes on specific screens
│ └─ Downsample images
│ Load at display size, not full resolution
│
├─ Memory not released after screen dismissal
│ └─ Debug object lifecycle
│ deinit not called = retain cycle
│
└─ Background memory pressure
└─ Respond to didReceiveMemoryWarning
Clear caches, release non-essential data
NEVER Do
View Identity
NEVER use indices as identifiers:
// ❌ Identity changes when array mutates
List(items.indices, id: \.self) { index in
ItemRow(item: items[index])
}
// Insert at index 0 → all views recreated!
// ✅ Use stable identifiers
List(items) { item in
ItemRow(item: item)
.id(item.id) // Stable across mutations
}
NEVER compute expensive values in body:
// ❌ Called on every render
var body: some View {
let sortedItems = items.sorted { $0.date > $1.date } // O(n log n) per render!
let filtered = sortedItems.filter { $0.isActive }
List(filtered) { item in
ItemRow(item: item)
}
}
// ✅ Compute in ViewModel or use computed property
@MainActor
class ViewModel: ObservableObject {
@Published var items: [Item] = []
var displayItems: [Item] {
items.filter(\.isActive).sorted { $0.date > $1.date }
}
}
State Management
NEVER use @StateObject for passed objects:
// ❌ Creates new instance on every parent update
struct ChildView: View {
@StateObject var viewModel: ChildViewModel // Wrong!
var body: some View { ... }
}
// ✅ Use @ObservedObject for passed objects
struct ChildView: View {
@ObservedObject var viewModel: ChildViewModel // Parent owns it
var body: some View { ... }
}
NEVER make everything @Published:
// ❌ Every property change triggers view updates
class ViewModel: ObservableObject {
@Published var items: [Item] = []
@Published var internalCache: [String: Data] = [:] // UI doesn't need this!
@Published var isProcessing = false // Maybe internal only
}
// ✅ Only publish what UI observes
class ViewModel: ObservableObject {
@Published var items: [Item] = []
@Published var isLoading = false
private var internalCache: [String: Data] = [:] // Not @Published
private var isProcessing = false // Private state
}
Memory Leaks
NEVER capture self strongly in escaping closures:
// ❌ Retain cycle — never deallocates
class ViewModel {
var timer: Timer?
func start() {
timer = Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true) { _ in
self.tick() // Strong capture!
}
}
}
// ✅ Weak capture + invalidation
class ViewModel {
var timer: Timer?
func start() {
timer = Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true) { [weak self] _ in
self?.tick()
}
}
deinit {
timer?.invalidate()
}
}
NEVER forget to remove observers:
// ❌ Leaks observer and potentially self
class ViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
NotificationCenter.default.addObserver(
self,
selector: #selector(handleNotification),
name: .userLoggedIn,
object: nil
)
// Never removed!
}
}
// ✅ Remove in deinit or use modern API
class ViewController: UIViewController {
private var observer: NSObjectProtocol?
override func viewDidLoad() {
super.viewDidLoad()
observer = NotificationCenter.default.addObserver(
forName: .userLoggedIn,
object: nil,
queue: .main
) { [weak self] _ in
self?.handleNotification()
}
}
deinit {
if let observer { NotificationCenter.default.removeObserver(observer) }
}
}
Image Loading
NEVER load full resolution for thumbnails:
// ❌ 4000×3000 image for 80×80 thumbnail
let image = UIImage(contentsOfFile: path) // Full resolution in memory!
imageView.image = image
// ✅ Downsample to display size
func downsampledImage(at url: URL, to size: CGSize) -> UIImage? {
let options: [CFString: Any] = [
kCGImageSourceShouldCache: false,
kCGImageSourceCreateThumbnailFromImageAlways: true,
kCGImageSourceThumbnailMaxPixelSize: max(size.width, size.height) * UIScreen.main.scale
]
guard let source = CGImageSourceCreateWithURL(url as CFURL, nil),
let cgImage = CGImageSourceCreateThumbnailAtIndex(source, 0, options as CFDictionary) else {
return nil
}
return UIImage(cgImage: cgImage)
}
NEVER cache images without limits:
// ❌ Unbounded memory growth
class ImageLoader {
private var cache: [URL: UIImage] = [:] // Grows forever!
func image(for url: URL) -> UIImage? {
if let cached = cache[url] { return cached }
let image = loadImage(url)
cache[url] = image // Never evicted
return image
}
}
// ✅ Use NSCache with limits
class ImageLoader {
private let cache = NSCache<NSURL, UIImage>()
init() {
cache.countLimit = 100
cache.totalCostLimit = 50 * 1024 * 1024 // 50 MB
}
func image(for url: URL) -> UIImage? {
if let cached = cache.object(forKey: url as NSURL) { return cached }
guard let image = loadImage(url) else { return nil }
cache.setObject(image, forKey: url as NSURL, cost: image.jpegData(compressionQuality: 1)?.count ?? 0)
return image
}
}
Heavy Operations
NEVER do heavy work on main thread:
// ❌ UI frozen during processing
func loadData() {
let data = try! Data(contentsOf: largeFileURL) // Blocks main thread!
let parsed = parseData(data) // Still blocking!
self.items = parsed
}
// ✅ Use background thread, update on main
func loadData() async {
let items = await Task.detached(priority: .userInitiated) {
let data = try! Data(contentsOf: largeFileURL)
return parseData(data)
}.value
await MainActor.run {
self.items = items
}
}
Essential Patterns
Efficient List View
struct EfficientListView: View {
let items: [Item]
var body: some View {
ScrollView {
LazyVStack(spacing: 12) { // Lazy = on-demand creation
ForEach(items) { item in
ItemRow(item: item)
.id(item.id) // Stable identity
}
}
}
}
}
// Equatable row prevents unnecessary updates
struct ItemRow: View, Equatable {
let item: Item
var body: some View {
HStack {
AsyncImage(url: item.imageURL) { image in
image.resizable().aspectRatio(contentMode: .fill)
} placeholder: {
Color.gray.opacity(0.3)
}
.frame(width: 60, height: 60)
.clipShape(RoundedRectangle(cornerRadius: 8))
VStack(alignment: .leading) {
Text(item.title).font(.headline)
Text(item.subtitle).font(.caption).foregroundColor(.secondary)
}
}
}
static func == (lhs: ItemRow, rhs: ItemRow) -> Bool {
lhs.item.id == rhs.item.id &&
lhs.item.title == rhs.item.title &&
lhs.item.subtitle == rhs.item.subtitle
}
}
Memory-Safe ViewModel
@MainActor
final class ViewModel: ObservableObject {
@Published private(set) var items: [Item] = []
@Published private(set) var isLoading = false
private var cancellables = Set<AnyCancellable>()
private var loadTask: Task<Void, Never>?
func load() {
loadTask?.cancel() // Cancel previous
loadTask = Task {
guard !Task.isCancelled else { return }
isLoading = true
defer { isLoading = false }
do {
let items = try await API.fetchItems()
guard !Task.isCancelled else { return }
self.items = items
} catch {
// Handle error
}
}
}
deinit {
loadTask?.cancel()
cancellables.removeAll()
}
}
Debounced Search
@MainActor
final class SearchViewModel: ObservableObject {
@Published var searchText = ""
@Published private(set) var results: [Item] = []
private var searchTask: Task<Void, Never>?
init() {
// Debounce search
$searchText
.debounce(for: .milliseconds(300), scheduler: DispatchQueue.main)
.removeDuplicates()
.sink { [weak self] text in
self?.performSearch(text)
}
.store(in: &cancellables)
}
private func performSearch(_ query: String) {
searchTask?.cancel()
guard !query.isEmpty else {
results = []
return
}
searchTask = Task {
do {
let results = try await API.search(query: query)
guard !Task.isCancelled else { return }
self.results = results
} catch {
// Handle error
}
}
}
}
Quick Reference
Instruments Selection
| Issue | Instrument | What to Look For | |-------|------------|------------------| | Slow UI | Time Profiler | Heavy main thread work | | Memory leak | Leaks | Leaked objects | | Memory growth | Allocations | Growing categories | | Battery | Energy Log | Wake frequency | | Network | Network | Request count, size | | Disk | File Activity | Excessive I/O | | GPU | Core Animation | Offscreen renders |
SwiftUI Performance Checklist
| Issue | Solution | |-------|----------| | Slow list scrolling | Use LazyVStack/LazyVGrid | | All items re-render | Stable IDs, Equatable rows | | Heavy body computation | Move to ViewModel | | Cascading @Published updates | Split or use computed | | Animation jank | Use drawingGroup() |
Memory Management
| Pattern | Prevent Issue | |---------|---------------| | [weak self] in closures | Retain cycles | | Timer.invalidate() in deinit | Timer leaks | | Remove observers in deinit | Observer leaks | | NSCache with limits | Unbounded cache growth | | Image downsampling | Memory spikes |
os_signpost for Custom Profiling
import os.signpost
let log = OSLog(subsystem: "com.app", category: .pointsOfInterest)
os_signpost(.begin, log: log, name: "DataProcessing")
// Expensive work
os_signpost(.end, log: log, name: "DataProcessing")
Red Flags
| Smell | Problem | Fix | |-------|---------|-----| | Indices as List IDs | Views recreated on mutation | Use stable identifiers | | Expensive body computation | Runs every render | Move to ViewModel | | @StateObject for passed object | Creates new instance | Use @ObservedObject | | Strong self in Timer/closure | Retain cycle | Use [weak self] | | Full-res images for thumbnails | Memory explosion | Downsample to display size | | Unbounded dictionary cache | Memory growth | Use NSCache with limits | | Heavy work without Task.detached | Blocks main thread | Use background priority |