performance-audit-standard

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:

Pattern	Complexity	Example
array.includes(x)	O(n)	Linear scan per check
array.find(x => ...)	O(n)	Linear scan
array.filter().map().filter()	O(3n) + 3 allocations	Multiple passes
Object.entries().find()	O(n)	Linear scan of object
array.sort() to find min/max	O(n log n)	Overkill for single value
Nested loops with includes	O(n*m)	Quadratic

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);
}