Skill: Startup (Launch) Optimization

Overview

Launch time is a first-impression metric Apple tracks (and surfaces in MetricKit/Xcode Organizer). It splits into pre-main (dynamic linking, static initializers) and post-main (your didFinishLaunching/first-frame work). The goal is a fast time to first usable frame: defer everything non-essential, do no blocking I/O or network on the launch path, and measure with the App Launch Instrument. Target well under ~400ms to first frame where possible.

Use Cases

• Slow cold launches hurting retention and App Store metrics.
• Heavy SDK initialization bloating startup.
• "White screen" before first content appears.

Best Practices

• Defer non-critical work off the launch path (Task after first frame, lazy init).
• No synchronous network/disk in didFinishLaunching or first view init.
• Lazy-initialize SDKs/analytics; initialize only what the first screen needs.
• Reduce pre-main cost: fewer dynamic frameworks, avoid heavy +load/static initializers.
• Measure with the App Launch Instrument and MetricKit; set a launch budget.

Anti-Patterns

• ❌ Initializing every SDK eagerly at launch.
• ❌ Synchronous network/database calls before first frame.
• ❌ Heavy work in the root view's init/onAppear.
• ❌ Many dynamic frameworks inflating pre-main linking.
• ❌ Optimizing launch without measuring it.

Checklist

• [ ] No synchronous network/disk on the launch path.
• [ ] Non-critical work deferred past first frame.
• [ ] SDKs lazily initialized; only first-screen deps loaded eagerly.
• [ ] Pre-main cost considered (frameworks, static init).
• [ ] Launch measured with a budget.

Swift Examples

@main
struct BankingApp: App {
    init() {
        // Keep launch lean: only what the first frame needs.
        CrashReporter.startMinimal()
    }
    var body: some Scene {
        WindowGroup {
            RootView()
                .task {
                    // Deferred, non-blocking startup work after first frame.
                    await AppBootstrap.shared.warmCaches()
                    Analytics.shared.initializeLazily()
                    await RemoteConfig.shared.refresh()
                }
        }
    }
}

enum AppBootstrap {
    static let shared = AppBootstrap()
    func warmCaches() async { /* prefetch off the critical path */ }
}

Common Interview Questions

• What are pre-main vs post-main launch phases?
• How do you measure launch time?
• Why avoid eager SDK initialization at launch?
• How do dynamic frameworks affect launch?
• What is "time to first frame" and why does it matter?

AI Implementation Notes

• Move startup work into a deferred .task/background after first frame; never block launch.
• Lazily initialize SDKs and analytics.
• Recommend measuring with the App Launch Instrument and setting a budget.
• Related: memory_optimization.md, battery_optimization.md.