He means that the performance intensive portion of Objective C programs (the tight loops, aka the 10% of the code that your code spends 90% of its time in, or whatever) are written in the subset of Objective C closer to C. E.g. they avoid dynamic dispatch in favor of static dispatch.

I'm not an Objective C programmer unless I can avoid it (C++ most days) so I can't really say if he's right or not. (Though, if I had to guess, I'd say he is)