But not all languages emit the same machine code. And I think performance is more sensitive to memory access than machine instructions, so I don't think this point is true.
The point is that a language can be faster than the language it is written in - all or for the most part - depending on how it is used. Saying "the theoretical performance of Java can never be faster than the theoretical performance of C" is a tautology and pretty useless - especially if in practice the Java libraries handle normal operations better than the average C programmer.
