I risk violating your request for no opinions but I would think that C++ would actually run slightly faster because it doesn’t have all the memory management and messaging overhead that goes with Objective-C. But, once compiled, it is native code in that it’s been compiled into a native instruction set, the exactly the same as Obj-C is. It’s not like Java which gets compiled into an intermediate instruction set (bytecode).
However, the performance differences, at this level, will really be the least of your concerns. Both languages are performant. It’s the game code that will determine most, if not all, of your performance.