It's not usually that hard because the higher level the decision, the more impactful it is on performance and the less a compiler will be able to bail you out.

It's not usually that hard because the higher level the decision, the more impactful it is on performance and the less a compiler will be able to bail you out. That's why you should start optimizing from the highest level and work your way down until the performance is acceptable. Only once you're at the point of optimizing low-level decisions does profiling really start to make sense. A silly example: it doesn't matter how much you optimize your bubble sort implementation, it'll lose out to the most naively written quicksort, which will lose out to keeping the data always sorted and inserting with binary search, which will lose out to finding out how to make the data inherently sorted, which will lose out to figuring out how to avoid the need for the data to be sorted in the first place. The fastest code is no code at all.