In many real world apps there just isn't a single chokepoint but overhead distributed over your entire object model. C++ gives you 8x the breathing room that Obj-C does before you have to start tearing your abstractions apart and coding direct. For a codebase of any real size that can easily be a dealbreaker.

This is why it's easy enough for advocates of python/php/ruby to say "just code the slow parts in c" but often far harder to actually do this without losing any advantage the higher level language bought you in the first place.