Actually not, it works a bit differently. In Objective-C 1, there was no indirection and one had to explicitly use the equivalent of PIMPL to hide private members from the header or avoid the fragile base class problem.

In Objective-C 2 the object meta-data contains a table of instance variable offsets. The dynamic linker can modify this table at load time so you can freely add both instance variables and methods to new revisions of a class.

So what is the deal? Well, when the holder object itself is heap allocated, pimpl is inefficient because every access will require dereferencing two pointers. Also you cannot put protected or virtual members in the internal pimpl class (then there would be no point to have those in the first place).

That being said, it is not like Objective-C is some pinnacle of performance - you cannot allocate objects on the heap, and the compiler doesn't perform any devirtualization. So for performance critical code you have to drop down to C or...C++ :)