This is a matter of experience. If I have a tight loop looping millions of times but I have a heap allocation inside, I can be 99% sure this would be slow, without looking at the profiler and if there is an easy way to avoid it without obfuscating the code (e.g simply taking it out of the loop, or using the stack instead of the heap), I'd just do it. Profiling is a waste of time in such cases.

Also, bottlenecks happen in surprising places, and allowing slow/bad code just because the profiler doesn't scream about it on the development machine is a recipe for surprising performance issues in the future. The slow code might become a problem when a user has slightly different task for the program.

I agree with you, too, that experience helps.

But I think the aphorism covers the case you’re describing too.

It’s possible that your loop is slow, but if you’re working on a program of sufficient size and complexity, you simply won’t know if it’s the bottleneck without using a profiler.

The purpose of the statement is to save you the trouble of 10xing performance of a function, so that it takes .1% of all execution time, instead of 1%. Do sufficiently complex programs, and without a profiler, you won’t really know if a loop is taking 1%, 10%, or 80%.