What "Performance" Even Means

Performance is two different numbers (latency and throughput), the rule that you measure before you optimize, and the truth that 'fast enough' is defined by a requirement and by what users actually feel.

1. Latency vs Throughput Latency is how long one operation takes; throughput is how many operations finish per second. They're different numbers, and improving one can quietly hurt the other.
2. Measure Before You Optimize The cardinal rule of performance: humans guess wrong about what's slow, so measure first, find the real bottleneck - the slowest stage that caps the whole system - and fix that, not what you assumed.
3. What "Fast Enough" Means Fast is relative - to a requirement and to human perception. The slowest requests (tail latency, the high percentiles) are what users actually feel, and optimizing past 'fast enough' is wasted effort.