| name | swiftui-performance-audit |
| description | Audit and improve SwiftUI runtime performance from code review and architecture. Use for requests to diagnose slow rendering, janky scrolling, high CPU/memory usage, excessive view updates, or layout thrash in SwiftUI apps, and to provide guidance for user-run Instruments profiling when code review alone is insufficient. |
SwiftUI Performance Audit
Quick start
Use this skill to diagnose SwiftUI performance issues from code first, then request profiling evidence when code review alone cannot explain the symptoms.
Workflow
- Classify the symptom: slow rendering, janky scrolling, high CPU, memory growth, hangs, or excessive view updates.
- If code is available, start with a code-first review using
references/code-smells.md.
- If code is not available, ask for the smallest useful slice: target view, data flow, reproduction steps, and deployment target.
- If code review is inconclusive or runtime evidence is required, guide the user through profiling with
references/profiling-intake.md.
- Summarize likely causes, evidence, remediation, and validation steps using
references/report-template.md.
1. Intake
Collect:
- Target view or feature code.
- Symptoms and exact reproduction steps.
- Data flow:
@State, @Binding, environment dependencies, and observable models.
- Whether the issue shows up on device or simulator, and whether it was observed in Debug or Release.
Ask the user to classify the issue if possible:
- CPU spike or battery drain
- Janky scrolling or dropped frames
- High memory or image pressure
- Hangs or unresponsive interactions
- Excessive or unexpectedly broad view updates
For the full profiling intake checklist, read references/profiling-intake.md.
2. Code-First Review
Focus on:
- Invalidation storms from broad observation or environment reads.
- Unstable identity in lists and
ForEach.
- Heavy derived work in
body or view builders.
- Layout thrash from complex hierarchies,
GeometryReader, or preference chains.
- Large image decode or resize work on the main thread.
- Animation or transition work applied too broadly.
Use references/code-smells.md for the detailed smell catalog and fix guidance.
Provide:
- Likely root causes with code references.
- Suggested fixes and refactors.
- If needed, a minimal repro or instrumentation suggestion.
3. Guide the User to Profile
If code review does not explain the issue, ask for runtime evidence:
- A trace export or screenshots of the SwiftUI timeline and Time Profiler call tree.
- Device/OS/build configuration.
- The exact interaction being profiled.
- Before/after metrics if the user is comparing a change.
Use references/profiling-intake.md for the exact checklist and collection steps.
4. Analyze and Diagnose
- Map the evidence to the most likely category: invalidation, identity churn, layout thrash, main-thread work, image cost, or animation cost.
- Prioritize problems by impact, not by how easy they are to explain.
- Distinguish code-level suspicion from trace-backed evidence.
- Call out when profiling is still insufficient and what additional evidence would reduce uncertainty.
5. Remediate
Apply targeted fixes:
- Narrow state scope and reduce broad observation fan-out.
- Stabilize identities for
ForEach and lists.
- Move heavy work out of
body into derived state updated from inputs, model-layer precomputation, memoized helpers, or background preprocessing. Use @State only for view-owned state, not as an ad hoc cache for arbitrary computation.
- Use
equatable() only when equality is cheaper than recomputing the subtree and the inputs are truly value-semantic.
- Downsample images before rendering.
- Reduce layout complexity or use fixed sizing where possible.
Use references/code-smells.md for examples, Observation-specific fan-out guidance, and remediation patterns.
6. 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.
Use references/report-template.md when formatting the final audit.
References
- Profiling intake and collection checklist:
references/profiling-intake.md
- Common code smells and remediation patterns:
references/code-smells.md
- Audit output template:
references/report-template.md
- 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
