swift-performance-pro

Swift Performance Pro

Find and fix performance problems with evidence, not guesses. Measure first.

When to use

• Janky scrolling, slow launch, high memory, or battery drain.
• Reviewing code for performance regressions.
• Planning an Instruments session.

Trigger: /swift-performance-pro.

Core principles

• Measure before optimizing — use Instruments (Time Profiler, Allocations, SwiftUI, Leaks), not intuition.
• Keep SwiftUI body cheap and pure; recomputation is frequent.
• Do heavy work off the main actor.
• Don't optimize what you haven't proven is hot.

SwiftUI render cost

Keep body free of side effects and heavy compute.

❌ Sorting on every render

var body: some View {
    List(items.sorted { $0.date > $1.date }) { Row($0) }   // re-sorts each pass
}

✅ Sort once in the model

// model exposes already-sorted `items`
var body: some View { List(model.items) { Row($0) } }

Narrow observation so unrelated changes don't redraw everything. Split big views into small subviews so SwiftUI can diff precisely.

Lists

• LazyVStack/List for long content, not eager VStack in a ScrollView.
• Stable Identifiable IDs — index-based identity forces full rebuilds.
• Avoid AnyView in row builders; it defeats SwiftUI's diffing.

Memory & retain cycles

❌ Strong self capture in a stored closure

manager.onUpdate = { self.refresh() }   // cycle: manager → closure → self

✅

manager.onUpdate = { [weak self] in self?.refresh() }

Verify deinit runs. Use Instruments → Leaks / Allocations to confirm.

Off the main actor

❌

let image = UIImage(data: hugeData)   // decode on main → dropped frames

✅

let image = await Task.detached { UIImage(data: hugeData) }.value

Downsample large images to the display size instead of loading full-resolution.

Launch time

• Defer non-critical work out of init/onAppear; load it in .task.
• Avoid synchronous disk/network on the launch path.
• Audit with Instruments → App Launch.

Common mistakes checklist

• Sorting/filtering/heavy compute inside body.
• Eager VStack where LazyVStack/List is needed.
• Index-based identity forcing full list rebuilds.
• AnyView in hot row builders.
• Strong self in stored closures (retain cycle).
• Image decode / disk / network on the main actor or launch path.
• Optimizing without an Instruments trace proving the hotspot.

Output format (when reviewing)

Per issue: file:line, the cost, before/after, and which Instrument confirms it. Lead with main-thread blockers.