# Performance — First Principles

## 1. The Principles

### P1. Measure First
No optimization without measurement. Intuition about performance is
usually wrong.

### P2. Critical Path Focus
Optimize what users actually wait for. The 95th percentile matters
more than the average.

### P3. Complexity Awareness
Algorithmic cost is known. Big-O is a design conversation, not an
afterthought.

### P4. Resource Bounds
Memory, CPU, I/O, network — all bounded. Unbounded growth is a bug.

### P5. Caching with Intent
Cache what is expensive, stable, and read often. Invalidation is
designed, not bolted on.

### P6. Lazy by Default
Compute only when needed. Pay only for what is used.

### P7. Async When Independent
Work that does not depend on other work runs in parallel.

### P8. Budget Discipline
Performance is a design constraint. The budget is set, not negotiated
after the fact.

### P9. Perceived Performance
What the user feels is what matters. A 200ms perceived response beats
a 50ms measured one with no feedback.

### P10. Regression Prevention
Performance tests catch what functional tests miss. The slow path
is tested as a path.