Investigating Performance

Overview

Performance work fails in a specific way: the engineer reads the code, forms an intuition about what must be slow, optimizes that, and ships a change that measurably does nothing. The bottleneck is somewhere else, because the bottleneck is almost always somewhere else.

Core principle: Measure, localize, change one thing, measure again. Intuition selects what to measure; it never selects what to fix.

Companion skills: systematic-debugging — same discipline applied to wrongness rather than slowness. Both refuse to act before the cause is located.

The Iron Law

NO OPTIMIZATION WITHOUT A MEASUREMENT THAT NAMES THE BOTTLENECK

You must be able to say "X accounts for N% of the time" before you change X. Not "X looks expensive." Not "X is O(n²)." A measurement, with a number.