Have you actually used the API? When it comes to submitting geometry and textures (some of the biggest bottlenecks), it's exactly the same as in C. You pass a pointer to your buffer of bytes and they get copied to the GPU for you.

When you're calling any library, that simple C call invariably goes somewhere else inside the library itself. For OpenGL this is because there's always hooks for introspection, or because the GPU driver wants to implement something themselves, etc. For many other libraries it's just because people don't feel like they've written "production quality code" unless it goes through a bunch of hoops and ends up in some method with five underscores in the name.

In Metal the Obj-C abstraction is part of the design and used to eliminate any other abstraction people would want to introduce. The objects you get back from the API are implemented straight in the GPU vendor code, and the debug tools can swap the methods out for extra validation, recording, etc.

Any overhead coming from objc_msgSend is minuscule compared to the gains from things like better serialization of GPU-bound tasks and not having to synchronize repeatedly with the CPU.

If you're worried about refcounting, use ARC (which you have to with Swift anyway). First, the compiler is very smart about optimizing away retain/release/autorelease calls whenever it can. Second, when those calls do have to be made, they're implemented using vtables, and never hit objc_msgSend() in the first place.

I think it is about time industry relevant OS vendors start to move away from C, kudos to Apple.