Correct me if i'm wrong, but Swift has static typing (with type inference, but still with a strict type on every variable), whereas Objective C has dynamic typing with optional type annotations. Having ubiquitous strong type information available should make it much easier to generate fast code.

Hm, maybe that's it. I thought typing was still dynamic because of the multiple return types.

guessing multiple return types are supported by just returning a tuple?

maybe it is accompanied by improvements in LLVM? Or optimizations in LLVM that are more feasible with Swift (it has immutable values).