Identifying Bottlenecks

The first rule of performance work: measure before you optimize.

Intuition about where a Java program spends its time is usually wrong. The JIT compiler, garbage collector, and caching all defeat hand-waving. Profile, find the real hotspot, fix that.

A bottleneck is the part of the system that limits overall throughput or latency.

Optimizing anything else gives no visible benefit. Amdahl's Law makes this precise: if 90% of time is in one method, speeding up the other 10% can never give more than an 11% improvement.