SwiftUI Performance Audit

Overview

Audit SwiftUI view performance end-to-end, from instrumentation and baselining to root-cause analysis and concrete remediation steps.

Workflow Decision Tree

• If the user provides code, start with "Code-First Review."

• If the user only describes symptoms, ask for minimal code/context, then do "Code-First Review."

• If code review is inconclusive, go to "Guide the User to Profile" and ask for a trace or screenshots.

1. Code-First Review

Collect:

• Target view/feature code.

• Data flow: state, environment, observable models.

• Symptoms and reproduction steps.

Focus on:

• View invalidation storms from broad state changes.

• Unstable identity in lists (id churn, UUID() per render).

• Heavy work in body (formatting, sorting, image decoding).

• Layout thrash (deep stacks, GeometryReader , preference chains).

• Large images without downsampling or resizing.

• Over-animated hierarchies (implicit animations on large trees).

Provide:

• Likely root causes with code references.

• Suggested fixes and refactors.

• If needed, a minimal repro or instrumentation suggestion.

2. Guide the User to Profile

Explain how to collect data with Instruments:

• Use the SwiftUI template in Instruments (Release build).

• Reproduce the exact interaction (scroll, navigation, animation).

• Capture SwiftUI timeline and Time Profiler.

• Export or screenshot the relevant lanes and the call tree.

Ask for:

• Trace export or screenshots of SwiftUI lanes + Time Profiler call tree.

• Device/OS/build configuration.

3. Analyze and Diagnose

Prioritize likely SwiftUI culprits:

• View invalidation storms from broad state changes.

• Unstable identity in lists (id churn, UUID() per render).

• Heavy work in body (formatting, sorting, image decoding).

• Layout thrash (deep stacks, GeometryReader , preference chains).

• Large images without downsampling or resizing.

• Over-animated hierarchies (implicit animations on large trees).

Summarize findings with evidence from traces/logs.

4. Remediate

Apply targeted fixes:

• Narrow state scope (@State /@Observable closer to leaf views).

• Stabilize identities for ForEach and lists.

• Move heavy work out of body (precompute, cache, @State ).

• Use equatable() or value wrappers for expensive subtrees.

• Downsample images before rendering.

• Reduce layout complexity or use fixed sizing where possible.

Common Code Smells (and Fixes)

Look for these patterns during code review.

Expensive formatters in body

var body: some View { let number = NumberFormatter() // slow allocation let measure = MeasurementFormatter() // slow allocation Text(measure.string(from: .init(value: meters, unit: .meters))) }

Prefer cached formatters in a model or a dedicated helper:

final class DistanceFormatter { static let shared = DistanceFormatter() let number = NumberFormatter() let measure = MeasurementFormatter() }

Computed properties that do heavy work

var filtered: [Item] { items.filter { $0.isEnabled } // runs on every body eval }

Prefer precompute or cache on change:

@State private var filtered: [Item] = [] // update filtered when inputs change

Sorting/filtering in body or ForEach

List { ForEach(items.sorted(by: sortRule)) { item in Row(item) } }

Prefer sort once before view updates:

let sortedItems = items.sorted(by: sortRule)

Inline filtering in ForEach

ForEach(items.filter { $0.isEnabled }) { item in Row(item) }

Prefer a prefiltered collection with stable identity.

Unstable identity

ForEach(items, id: .self) { item in Row(item) }

Avoid id: .self for non-stable values; use a stable ID.

Image decoding on the main thread

Image(uiImage: UIImage(data: data)!)

Prefer decode/downsample off the main thread and store the result.

Broad dependencies in observable models

@Observable class Model { var items: [Item] = [] }

var body: some View { Row(isFavorite: model.items.contains(item)) }

Prefer granular view models or per-item state to reduce update fan-out.

5. Verify

Ask the user to re-run the same capture and compare with baseline metrics. Summarize the delta (CPU, frame drops, memory peak) if provided.

Outputs

Provide:

• A short metrics table (before/after if available).

• Top issues (ordered by impact).

• Proposed fixes with estimated effort.

References

Add Apple documentation and WWDC resources under references/ as they are supplied by the user.

• Optimizing SwiftUI performance with Instruments: references/optimizing-swiftui-performance-instruments.md

• Understanding and improving SwiftUI performance: references/understanding-improving-swiftui-performance.md

• Understanding hangs in your app: references/understanding-hangs-in-your-app.md

• Demystify SwiftUI performance (WWDC23): references/demystify-swiftui-performance-wwdc23.md