Premature optimization is time spent not optimizing things that actually matter. In my own circles, it's the accusation legitimately levied against the clowns

Premature optimization is time spent not optimizing things that actually matter. In my own circles, it's the accusation legitimately levied against the clowns who simultaniously write macros instead of functions out of fears that the compiler could theoretically ignore a __forceinline keyword - or if you're lucky, optimizing some microbenchmark while pessimizing overall program performance - and in the next breath writing O(scary) code that'll be invoked with large enough N to demolish any savings from micro-optimizations. That is, despite optimizing, these are people who, despite an obsession with performance, are still treating the performance of their overall programs as an afterthought. Because they don't actually care about performance as much as they say: they care about showing off and flexing their coding skills by making a mess. Well, the phrase itself doesn't help guide these people towards real performance engineering, but they're typically stubborn enough to ignore the phrase, and must instead be goaded into using a profiler or generating benchmarks, which can then be picked apart when they optimize away the actual execution of everything at compile time, teaching by way of concrete example at least a little bit about generating better microbenchmarks, which they can use to show off and flex more effectively instead.