Performance Measurement Discipline

When to use

• Any request that starts with "make this faster" — before touching code, you need a measurement of where the time actually goes.
• Writing a benchmark or microbenchmark that must produce trustworthy numbers.
• Choosing a timing tool and interpreting its output (perf stat/perf record, hyperfine, clock_gettime, QueryPerformanceCounter).
• Deciding whether an optimization worked: before/after on a fixed baseline.
• Explaining why code "should be fast" but is not (profile-first reasoning).

When not to use