I can't speak for other people's code, but I regularly profile and read the machine code generated by the Swift compiler, at least for my hot loops. If you know what you are doing (use the right annotations and optimizer flags), even fairly generic code often compiles down to something that comes very close to what an optimizing C compiler would generate. Sometimes it generates even faster code, because the Swift calling convention can make better use of available registers. Sure, there are situations where it generates less optimal code, but generally generic idiomatic Swift is on a different level than a (non-profiling) compiler for idiomatic Objective-C can ever come close to. That's my experience.