The last benchmarks[1] I saw for Swift showed it's generally pretty slow without optimisations on, but various differences in the language from Objective C mean the compiler can optimise it much more, so generally it's faster than Objective C (and actually comparable to pure C at times[2]).
Those benchmarks (part 2) were ludicrously flawed, comparing native integers in Swift with NSNumber objects in Objective-C.
The fact that the standard library qsort() routine, which uses function pointers and pointers to the objects to be sorted in the array has overhead is also obvious, a truly native implementation is still much better[1]
[1] http://www.jessesquires.com/apples-to-apples-part-two [2] http://www.jessesquires.com/apples-to-apples-part-three/