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skills/expertise/iphone-apps/workflows/optimize-performance.md

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+# Workflow: Optimize iOS App Performance
+
+<required_reading>
+**Read NOW:**
+1. references/performance.md
+2. references/cli-observability.md
+</required_reading>
+
+<philosophy>
+Measure first, optimize second. Never optimize based on assumptions.
+</philosophy>
+
+<process>
+## Step 1: Define the Problem
+
+Ask:
+- What feels slow?
+- Startup? Scrolling? Specific action?
+- When did it start?
+
+## Step 2: Measure
+
+**CPU Profiling:**
+```bash
+xcrun xctrace record \
+  --template 'Time Profiler' \
+  --device 'iPhone 16' \
+  --attach AppName \
+  --output profile.trace
+```
+
+**Memory:**
+```bash
+xcrun xctrace record --template 'Allocations' ...
+```
+
+**Launch time:**
+```bash
+# Add DYLD_PRINT_STATISTICS=1 to scheme environment
+```
+
+## Step 3: Identify Bottlenecks
+
+Look for:
+- Functions with high "self time"
+- Main thread blocking
+- Repeated expensive operations
+- Large allocations
+
+**SwiftUI re-renders:**
+```swift
+var body: some View {
+    let _ = Self._printChanges()
+    // ...
+}
+```
+
+## Step 4: Common Optimizations
+
+### Main Thread
+```swift
+// Bad
+let data = expensiveWork() // blocks UI
+
+// Good
+let data = await Task.detached { expensiveWork() }.value
+```
+
+### SwiftUI
+```swift
+// Bad - rebuilds everything
+struct BigView: View {
+    @State var a, b, c, d, e
+}
+
+// Good - isolated state
+struct BigView: View {
+    var body: some View {
+        SubViewA() // has own @State
+        SubViewB() // has own @State
+    }
+}
+```
+
+### Lists
+```swift
+// Use LazyVStack for long lists
+ScrollView {
+    LazyVStack {
+        ForEach(items) { ... }
+    }
+}
+```
+
+### Images
+```swift
+AsyncImage(url: url) { image in
+    image.resizable()
+} placeholder: {
+    ProgressView()
+}
+```
+
+## Step 5: Measure Again
+
+Did it improve? If not, revert.
+
+## Step 6: Performance Tests
+
+```swift
+func testScrollPerformance() {
+    measure(metrics: [XCTCPUMetric(), XCTMemoryMetric()]) {
+        // scroll simulation
+    }
+}
+```
+</process>
+
+<targets>
+| Metric | Target | Unacceptable |
+|--------|--------|--------------|
+| Launch | < 1s | > 2s |
+| Scroll | 60 fps | < 30 fps |
+| Response | < 100ms | > 500ms |
+</targets>