Performance Audit Standard

Methodology for identifying and fixing performance bottlenecks.

Audit Process

Step 1: Identify Hot Paths

Find code that runs frequently or processes large datasets:

• Request handlers (every HTTP request)
• Event processors (every WebSocket message)
• Loop bodies processing collections (per-item)
• Interval timers (every N seconds)
• Middleware/interceptors (every request)

Ask: "How many times per second does this code execute?" and "What's the typical input size?"

Step 2: Analyze Complexity

For each hot path, determine actual Big O:

Step 3: Apply Fixes

Common Performance Anti-Patterns

1. O(n) Membership Test → Use Set

// BAD: O(n) per check, O(n*m) in a loop
const newIds = currentIds.filter(x => !savedIds.includes(x));

// GOOD: O(1) per check, O(n+m) total
const savedSet = new Set(savedIds);
const newIds = currentIds.filter(x => !savedSet.has(x));

Impact: 1000x on large collections (1000 items: 1M comparisons → 1K).

2. O(n log n) Selection → Single-Pass

// BAD: filter + sort + take first = O(n) + O(n log n) + O(1)
const available = tokens.filter(t => !t.expired);
available.sort((a, b) => a.lastUsed - b.lastUsed);
const best = available[0];

// GOOD: Single O(n) pass
let best = null;
for (const t of tokens) {
  if (t.expired) continue;
  if (!best || t.lastUsed < best.lastUsed) best = t;
}

Impact: 10-50x faster, zero intermediate arrays.

3. Linear Lookup → Map Index

// BAD: O(n) per lookup
function findToken(value) {
  return tokens.find(t => t.value === value);
}

// GOOD: O(1) per lookup
const tokenIndex = new Map(tokens.map(t => [t.value, t]));
function findToken(value) {
  return tokenIndex.get(value);
}

Impact: 10-20x per lookup. Maintain index on add/remove.

4. Per-Request Parsing → Cache on Init

// BAD: Parse every time
function getProxy() {
  const proxies = proxySource.split(/\r?\n/);                    // regex split
  const parts = proxies[random()].split(":");                     // string split
  return `http://${parts[2]}:${parts[3]}@${parts[0]}:${parts[1]}`; // format
}

// GOOD: Parse once, cache formatted URLs
class ProxyPool {
  constructor(source) {
    this.urls = source.split(/\r?\n/).filter(Boolean).map(line => {
      const [h, p, u, pw] = line.split(":");
      return `http://${u}:${pw}@${h}:${p}`;
    });
  }
  random() { return this.urls[Math.floor(Math.random() * this.urls.length)]; }
}

Impact: 100-1000x. Regex + string operations are expensive per-call.

5. Sync File I/O → Async

// BAD: Blocks event loop 5-50ms
fs.writeFileSync(path, JSON.stringify(data, null, 2));

// GOOD: Non-blocking
await fs.promises.writeFile(path, JSON.stringify(data, null, 2));

// BETTER: Skip if unchanged
const hash = crypto.createHash("md5").update(JSON.stringify(data)).digest("hex");
if (hash !== this.lastHash) {
  await fs.promises.writeFile(path, JSON.stringify(data, null, 2));
  this.lastHash = hash;
}

6. Per-Request Client Creation → Singleton

// BAD: New TLS handshake + connection pool per call
async fn send(body: Value) {
    let client = reqwest::Client::builder().build().unwrap();
    client.post(url).json(&body).send().await;
}

// GOOD: Build once, reuse forever
static CLIENT: Lazy<reqwest::Client> = Lazy::new(|| {
    reqwest::Client::builder().timeout(Duration::from_secs(10)).build().unwrap()
});

7. Lock Held Across I/O → Clone and Release

// BAD: Lock held during disk write
let mut data = state.data.write().await;
data.insert(key, value);
let json = serde_json::to_string(&*data).unwrap();
std::fs::write(path, json); // DISK I/O WHILE LOCKED

// GOOD: Clone, release, then write
let json = {
    let mut data = state.data.write().await;
    data.insert(key, value);
    serde_json::to_string(&*data).unwrap()
}; // Lock dropped here
tokio::fs::write(path, json).await;

8. Repeated Computation in Loops → Hoist

// BAD: toLowerCase() called 1000x per event
for (const seat of seats) {
  const name = seat.section.toLowerCase();
  const match = filters.find(f => name.includes(f.keyword.toLowerCase()));
}

// GOOD: Pre-compute outside loop
const lowerKeywords = filters.map(f => ({
  ...f,
  lowerKeyword: f.keyword.toLowerCase(),
}));
for (const seat of seats) {
  const name = seat.section.toLowerCase();
  const match = lowerKeywords.find(f => name.includes(f.lowerKeyword));
}

9. Multiple Array Passes → Single Pass

// BAD: 3 passes, 3 intermediate arrays
const resale = picks.filter(p => p.resaleId);
const normal = picks.filter(p => !p.resaleId && !isFiltered(p));
const combined = [...resale.map(transform), ...normal.map(transform)];

// GOOD: Single pass
const results = [];
for (const p of picks) {
  if (p.resaleId) {
    results.push(transformResale(p));
  } else if (!isFiltered(p)) {
    results.push(transformNormal(p));
  }
}

10. Database: N+1 Queries

// BAD: 1 + N queries (1 list, N detail loads)
const users = await db.user.findMany();
for (const u of users) {
  u.orders = await db.order.findMany({ where: { userId: u.id } });
}

// GOOD: 2 queries, joined in app
const users = await db.user.findMany({ include: { orders: true } });

How to find them: log queries per request, set a budget, fail tests if exceeded. Most ORMs have an include / with / Preload to fetch in one round trip.

11. Database: Missing Indexes

EXPLAIN ANALYZE SELECT * FROM events WHERE user_id = $1 ORDER BY created_at DESC LIMIT 50;
-- Seq Scan ... rows=2_500_000  → missing index
CREATE INDEX events_user_id_created_at_idx ON events (user_id, created_at DESC);

Audit: every column in WHERE, JOIN, or ORDER BY on a hot query needs an index. Composite index order matches query order (equality columns first, then range).

12. Frontend: Bundle Size and Core Web Vitals

Common wins: code-split routes, lazy-load below-fold images (loading="lazy"), tree-shake icon libs, swap large polyfills.

Priority Quick-Wins Matrix

Report findings in this format:

Sort by impact/effort ratio. Estimate combined CPU reduction.

Language-Specific Tools

JavaScript/Node.js

• node --prof + node --prof-process for V8 profiling
• clinic.js (doctor, flame, bubbleprof)
• 0x for flame graphs
• process.memoryUsage() for heap monitoring

Rust

• cargo flamegraph for flame graphs
• criterion for microbenchmarks
• dhat for heap profiling
• tokio-console for async task inspection
• perf (Linux) or Instruments (macOS)

Go

• pprof (CPU + memory profiles)
• go test -bench
• trace for goroutine analysis

Swift/iOS

• Instruments: Time Profiler, Allocations, Leaks
• Xcode Memory Graph Debugger
• os_signpost for custom measurements

When NOT to Optimize

• Code runs < 100 times/second with < 100 items
• Optimization adds significant complexity for < 2x gain
• The bottleneck is I/O bound (network, disk), not CPU
• Profiling shows the code isn't actually hot
• "Profile first, optimize second": always measure before and after