App performance problem?
├─ App feels slow or lags (UI interactions stall, scrolling stutters)
│  └─ → Use Time Profiler (measure CPU usage)
├─ Memory grows over time (Xcode shows increasing memory)
│  └─ → Use Allocations (measure object creation)
├─ Data loading is slow (parsing, database queries, API calls)
│  └─ → Use Core Data instrument (if using Core Data)
│  └─ → Use Time Profiler (if it's computation)
└─ Battery drains fast (device gets hot, depletes in hours)
   └─ → Use Energy Impact (measure power consumption)

open -a Instruments

Time Profiler Results

MyViewController.viewDidLoad() – 500ms (40% of total)
  ├─ DataParser.parse() – 350ms
  │  └─ JSONDecoder.decode() – 320ms
  └─ UITableView.reloadData() – 150ms

// ❌ WRONG: Profile shows DataParser.parse() is 80% CPU
// Conclusion: "DataParser is slow, let me optimize it"

// ✅ RIGHT: Check what DataParser is calling
// If JSONDecoder.decode() is doing 99% of the work,
// optimize JSON decoding, not DataParser

// ❌ WRONG: Profile app in Simulator
// Simulator CPU is different than real device
// Results don't reflect actual device performance

// ✅ RIGHT: Profile on actual device
// Device settings: Developer Mode enabled, Xcode attached

// ❌ WRONG: Profile entire app startup
// Sees 2000ms startup time, many functions involved

// ✅ RIGHT: Profile just the slow part
// "App feels slow when scrolling" → profile only scrolling
// Separate concerns: startup slow vs interaction slow

open -a Instruments

// ❌ WRONG: Memory went from 100MB to 500MB
// Conclusion: "There's a leak, memory keeps growing!"

// ✅ RIGHT: Check what caused the growth
// Loaded 1000 images (normal)
// Cached API responses (normal)
// User has 5000 contacts (normal)
// Memory is being used correctly

// ❌ WRONG: Allocations shows 1000 UIImages in memory
// Conclusion: "Memory leak, too many images!"

// ✅ RIGHT: Check if this is intentional caching
// ImageCache holds up to 1000 images by design
// When memory pressure happens, cache is cleared
// Normal behavior

// ❌ WRONG: Record for 5 seconds, see 200MB
// Conclusion: "App uses 200MB, optimize memory"

// ✅ RIGHT: Record for 2-3 minutes, see full lifecycle
// Load data: 200MB
// Navigate away: 180MB (20MB still cached)
// Navigate back: 190MB (cache reused)
// Real baseline: ~190MB at steady state

Edit Scheme → Run → Arguments Passed On Launch
Add: -com.apple.CoreData.SQLDebug 1

CoreData: sql: SELECT ... FROM tracks WHERE artist = ? (time: 0.015s)
CoreData: sql: SELECT ... FROM albums WHERE id = ? (time: 0.002s)

❌ BAD: Loading 100 tracks, then querying album for each
SELECT * FROM tracks (time: 0.050s) → 100 tracks
SELECT * FROM albums WHERE id = 1 (time: 0.005s)
SELECT * FROM albums WHERE id = 2 (time: 0.005s)
SELECT * FROM albums WHERE id = 3 (time: 0.005s)
... 97 more queries
Total: 0.050s + (100 × 0.005s) = 0.550s

✅ GOOD: Fetch tracks WITH album relationship (eager loading)
SELECT tracks.*, albums.* FROM tracks
LEFT JOIN albums ON tracks.albumId = albums.id
(time: 0.050s)
Total: 0.050s

open -a Instruments

Core Data Results

Fetch Requests: 102
Average Fetch Time: 12ms
Slow Fetch: "SELECT * FROM tracks" (180ms)

Fault Fires: 5000
  → Object accessed, requires fetch from database
  → Should use prefetching

// ❌ WRONG: Fetch tracks, then access album for each
let tracks = try context.fetch(Track.fetchRequest())
for track in tracks {
    print(track.album.title)  // Fires individual query for each
}
// Total: 1 + N queries

// ✅ RIGHT: Fetch with relationship prefetching
let request = Track.fetchRequest()
request.returnsObjectsAsFaults = false
request.relationshipKeyPathsForPrefetching = ["album"]
let tracks = try context.fetch(request)
for track in tracks {
    print(track.album.title)  // Already loaded
}
// Total: 1 query

// ❌ WRONG: Fetch 50,000 records all at once
let request = Track.fetchRequest()
let allTracks = try context.fetch(request)  // Huge memory spike

// ✅ RIGHT: Batch fetch in chunks
let request = Track.fetchRequest()
request.fetchBatchSize = 500  // Fetch 500 at a time
let allTracks = try context.fetch(request)  // Memory efficient

// ❌ WRONG: Keep all objects in memory
let request = Track.fetchRequest()
request.returnsObjectsAsFaults = false  // Keep all in memory
let allTracks = try context.fetch(request)  // 50,000 objects
// Memory spike if you don't use all of them

// ✅ RIGHT: Use faults (lazy loading)
let request = Track.fetchRequest()
// request.returnsObjectsAsFaults = true (default)
let allTracks = try context.fetch(request)  // Just references
// Only load objects you actually access

Time Profiler: JSONDecoder.decode() = 500ms
Allocations: Memory = 250MB (normal for app size)

Result: App is slow AND memory is fine
Action: Optimize JSON decoding (not memory)

// Time Profiler: Launch Instruments
open -a Instruments

// Core Data: Enable SQL logging
// Edit Scheme → Run → Arguments Passed On Launch
-com.apple.CoreData.SQLDebug 1

// Allocations: Check persistent objects
Instruments → Allocations → Statistics → sort "Persistent"

// Memory warning: Simulate pressure
Xcode → Debug → Simulate Memory Warning

// Energy Impact: Profile battery drain
Instruments → Energy Impact template

// Network Link Conditioner: Simulate 3G
System Preferences → Network Link Conditioner → 3G profile

Performance problem?
├─ App feels slow/laggy?
│  └─ → Time Profiler (measure CPU)
├─ Memory grows over time?
│  └─ → Allocations (find object growth)
├─ Data loading is slow?
│  └─ → Core Data instrument (if using Core Data)
│  └─ → Time Profiler (if computation slow)
└─ Battery drains fast?
   └─ → Energy Impact (measure power)

undefined

**What you see in console**:

**Diagnosis using the skill**:
- Fetching 100 albums, then individual query for each album's artist = **N+1 query problem** (Core Data Deep Dive, lines 302-325)

**Fix**:
```swift
// ❌ WRONG: Each album access triggers separate artist query
let request = Album.fetchRequest()
let albums = try context.fetch(request)
for album in albums {
    print(album.artist.name)  // Extra query for each
}

// ✅ RIGHT: Prefetch the relationship
let request = Album.fetchRequest()
request.returnsObjectsAsFaults = false
request.relationshipKeyPathsForPrefetching = ["artist"]
let albums = try context.fetch(request)
for album in albums {
    print(album.artist.name)  // Already loaded
}

open -a Instruments

2. **Record the stall**:

3. **Examine results**:

4. **Apply the skill** (lines 173-175):

5. **Fix the right thing**:
```swift
// ❌ WRONG: Thread everything
DispatchQueue.global().async { loadJSON() }

// ✅ RIGHT: Thread only the slow part
func loadJSON() {
    let data = parseJSON()  // 50ms, fine on main

    // Move ONLY the slow part to background
    DispatchQueue.global().async {
        let images = loadImages()  // 1150ms, now background
        DispatchQueue.main.async {
            updateUI(with: images)
        }
    }
}

User loads data → memory grows to 400MB
User navigates around → memory stays at 400MB
User goes to Settings → memory at 400MB
User comes back → memory at 400MB

Persistent Objects:
- UIImage: 1200 instances (300MB) ← Large count
- NSString: 5000 instances (4MB)
- CustomDataModel: 800 instances (15MB)

Memory growing = apps using data users asked for
Memory dropping under pressure = cache working correctly
Memory staying high indefinitely = possible leak

// ✅ This is working correctly
let imageCache = NSCache<NSString, UIImage>()
// Holds up to 1200 images by design
// Clears when system memory pressure happens
// No leak