That’s a non-issue for the vast majority of people. As an example, I have written games in Objective-C for the first iPhones. In a game running at 50 fps you have about 20 ms to get a single frame out, and still I could freely use message passing in the inner game loop without giving it a thought. See also some older measurements by Mike Ash: http://goo.gl/DBTPE.
An Obj-C message is still about 5x slower than a C++ virtual function call, but even that is too slow for inner loops of expensive algorithms. For example, I'm doing DSP at 44.1k ops per second and I can only afford about one virtual function call for every sixteen samples but I can put my samples in an STL vector with no access overhead vs a raw array.
It's true that most people don't need the performance you can get with C++ but if you do it's still really your only serious option.
According to Mike’s measurements I linked above a cached Objective-C message send is faster than C++ virtual method call, so in a loop both languages should come pretty close. You can also get a pointer to the function implementing a method and call it directly. But I think we now understand each other – yes, Objective-C is not as fast as C++ in some cases, but those cases only matter to a very small number of people.
Also, it's _really_ easy to optimize those inner loops if you need to in Objective-C. If message passing is too slow, just turn it into some C function calls and you're done.
