Perf

Performance Mode

Recommended model tier: smart (opus) - this skill requires complex reasoning

Systematic approach to identifying and fixing performance issues.

Prerequisites

Before starting:

• Identify the specific operation or endpoint that is slow
• Understand what "fast enough" means (target latency, throughput)
• Ensure you can measure performance reproducibly

Workflow

Step 1: Establish Baseline Measurement

Never optimize without data. Measure current performance:

# Node.js - simple timing
time node script.js

# Node.js - CPU profiling
node --cpu-prof script.js
# Creates CPU.*.cpuprofile - analyze in Chrome DevTools

# Go - benchmarks
go test -bench=. -benchmem ./...

# API endpoint (ENDPOINT_URL is the URL under test)
curl -w "@curl-format.txt" -o /dev/null -s "$ENDPOINT_URL"

Record baseline metrics:

• Execution time (p50, p95, p99 if available)
• Memory usage
• Number of operations per second
• Number of I/O operations

Step 2: Identify Hotspots

Find where time is being spent:

# Node.js profiling
node --cpu-prof app.js
# Then load .cpuprofile in Chrome DevTools > Performance

# Go profiling
go test -cpuprofile=cpu.prof -bench=.
go tool pprof -http=:8080 cpu.prof

# Get structural overview of suspect files (signatures + line ranges, not full content)
mcp__plugin_aide_aide__code_outline file="path/to/hotspot.ts"

# Find functions/classes in suspect area by name
mcp__plugin_aide_aide__code_search query="processData" kind="function"

# Find all callers of a hot function
mcp__plugin_aide_aide__code_references symbol="processData"

# Search for expensive patterns in code bodies (Grep is better here)
Grep for ".forEach(", ".map(", ".filter("    # Loop/iteration patterns
Grep for "SELECT", "find(", "query("         # Database queries
Grep for "fetch(", "axios", "http.Get"       # Network calls
Grep for "setTimeout", "setInterval"         # Timers
Grep for "JSON.parse", "JSON.stringify"      # Serialization

Note: code_search finds function/class/type definitions by name. For patterns inside function bodies (loops, queries, call chains), use Grep.

After identifying hotspot functions via profiling and search, use Read with offset/limit to read specific functions (use line numbers from code_outline).

Step 3: Analyze Performance Patterns

Look for these common issues:

Issue	Pattern	Solution
N+1 queries	Loop containing DB call	Batch/eager load
Repeated computation	Same calculation in loop	Memoize/cache
Large allocations	Creating objects in loop	Reuse/pool objects
Blocking I/O	Sync file/network ops	Make async/concurrent
Missing indexes	Slow DB queries	Add database indexes
Unnecessary work	Processing unused data	Filter/skip early
Serial execution	Sequential independent ops	Parallelize

Step 4: Apply Optimizations

Priority order (highest impact first):

1. Algorithmic improvements - O(n^2) -> O(n log n)
2. Reduce I/O - Batch requests, add caching
3. Parallelize - Concurrent operations
4. Reduce allocations - Reuse objects, pre-allocate
5. Micro-optimizations - Only as last resort

Make one optimization at a time and measure after each.

Step 5: Measure After Each Change

# Same measurement as baseline
time node script.js
go test -bench=. -benchmem ./...

Compare:

• Did the metric improve?
• By how much (percentage)?
• Any negative side effects?

If no improvement: Revert and try different approach.

Step 6: Verify No Regressions

# Run all tests
npm test
go test ./...

# Check for correctness
# Ensure output is still correct after optimization

Failure Handling

Situation	Action
Cannot measure reliably	Increase sample size, reduce variance sources
Optimization made it slower	Revert, analyze why, profile more carefully
Optimization broke tests	Fix tests or revert if behavior changed
Bottleneck is external	Document, consider caching, async processing
Memory improved but CPU worse	Evaluate trade-off for use case

Common Optimizations

JavaScript/TypeScript

// BAD: N+1 queries
for (const user of users) {
  const posts = await db.getPosts(user.id);
}

// GOOD: Batch query
const userIds = users.map((u) => u.id);
const posts = await db.getPostsForUsers(userIds);

// BAD: Repeated work
const typeA = items.filter((x) => x.type === "a").map((x) => x.value);
const typeB = items.filter((x) => x.type === "b").map((x) => x.value);

// GOOD: Single pass
const grouped = { a: [], b: [] };
for (const x of items) {
  if (x.type in grouped) grouped[x.type].push(x.value);
}

// BAD: Serial async
const result1 = await fetch(url1);
const result2 = await fetch(url2);

// GOOD: Parallel async
const [result1, result2] = await Promise.all([fetch(url1), fetch(url2)]);

Go

// BAD: Allocation in loop
var result []T
for _, item := range items {
    result = append(result, process(item))
}

// GOOD: Pre-allocate
result := make([]T, 0, len(items))
for _, item := range items {
    result = append(result, process(item))
}

// BAD: String concatenation
s := ""
for _, item := range items {
    s += item
}

// GOOD: Builder
var b strings.Builder
for _, item := range items {
    b.WriteString(item)
}

SQL

-- BAD: Missing index on frequently queried column
SELECT * FROM users WHERE email = '[email protected]';

-- GOOD: Add index
CREATE INDEX idx_users_email ON users(email);

-- BAD: SELECT *
SELECT * FROM users;

-- GOOD: Select only needed columns
SELECT id, name, email FROM users;

-- BAD: Query in loop
-- for each user: SELECT * FROM posts WHERE user_id = ?

-- GOOD: Single batch query
SELECT * FROM posts WHERE user_id IN (?, ?, ?);

MCP Tools

• mcp__plugin_aide_aide__code_outline - Start here. Get collapsed file skeleton to identify functions before reading
• mcp__plugin_aide_aide__code_search - Find function/class/type definitions by name
• mcp__plugin_aide_aide__code_symbols - List all symbol definitions in a file
• mcp__plugin_aide_aide__code_references - Find all callers of a hot function (exact name match)
• mcp__plugin_aide_aide__memory_search - Check past performance decisions
• Grep - Find code patterns in bodies: loops, queries, call chains, string literals

Profiling Commands Reference

Node.js

# CPU profiling
node --cpu-prof app.js
# Produces .cpuprofile file

# Memory profiling
node --heap-prof app.js
# Produces .heapprofile file

# Clinic.js for analysis
npx clinic doctor -- node app.js
npx clinic flame -- node app.js

Go

# CPU profiling
go test -cpuprofile=cpu.prof -bench=.
go tool pprof -http=:8080 cpu.prof

# Memory profiling
go test -memprofile=mem.prof -bench=.
go tool pprof -http=:8080 mem.prof

# Execution trace
go test -trace=trace.out -bench=.
go tool trace trace.out

Browser

• DevTools -> Performance -> Record
• DevTools -> Memory -> Heap snapshot
• Lighthouse for overall page performance

Verification Criteria

Before completing:

• Baseline measurement recorded
• Improvement quantified (percentage)
• All tests still pass
• No correctness regressions
• Memory usage acceptable

Output Format

## Performance Analysis: [Operation/Endpoint Name]

### Baseline

- Execution time: 450ms (p50), 680ms (p95)
- Memory: 125MB peak
- Database queries: 150

### Hotspots Identified

1. `db.getUsers()` - 300ms (67% of total)
2. `processData()` - 100ms (22% of total)
3. `formatOutput()` - 50ms (11% of total)

### Optimizations Applied

1. Batched user queries - 300ms -> 50ms
2. Memoized processData for repeated calls - 100ms -> 5ms

### Results

- Execution time: 450ms -> 105ms (77% faster)
- Memory: 125MB -> 80MB (36% reduction)
- Database queries: 150 -> 3 (98% reduction)

### Verification

- All tests: PASS
- Output correctness: VERIFIED