Performance Profiling

Core Principle

Measure first. Optimize second. Verify third.

Never optimize based on intuition. Profile to find actual bottlenecks.

The Process

1. Establish Baseline

# Node.js
node --prof app.js
node --prof-process isolate-*.log > profile.txt

# Browser
Performance tab → Record → Analyze

2. Identify Bottlenecks

Look for:

• Functions with high "self time"
• Unexpected call counts
• Memory allocations in hot paths
• N+1 queries

3. Form Hypothesis

"Function X is slow because Y"

4. Optimize Minimally

One change at a time:

• Caching
• Algorithm improvement
• Batch operations
• Lazy loading

5. Verify Improvement

Compare against baseline. Must be measurably better.

Common Bottlenecks

Symptom	Likely Cause
Slow page load	Large bundle, blocking resources
Slow API	N+1 queries, missing indexes
Memory growth	Leaks, unbounded caches
CPU spikes	Inefficient loops, regex

Tools

Node.js:

• --prof flag
• clinic.js
• 0x flame graphs

Browser:

• DevTools Performance
• Lighthouse
• WebPageTest

Database:

• EXPLAIN ANALYZE
• Query logs
• Connection pool stats

Red Flags

• Optimizing without profiling
• "I think this is slow"
• Premature optimization
• Micro-optimizations in cold paths