Honestly, unless they release the source to the tests they are using I wouldn't trust anything Apple has to say on the subject. Remember, this is the company which switched from PPC to Intel based on power concerns when 6 months earlier they were saying that Intel sucked and actually torched the Intel bunny in a commercial. I would like to see hard irrefutable proof that Swift is faster than ObjC in more categories than just sorting.
Additionally you have to question any stats which are released at WWDC since they have the smell of marketing all over them.
All of that being said I haven't run any tests between swift and ObjC myself, but from what I know swift has it's own LLVM IR extensions and it is possible that more optimization is being done at compile time than in ObjC.