Comment by RussianCow

You're missing the point. The fact that a language compiles down to C doesn't mean it compiles down to efficient C. At the simplest level, the compiled code could add a bunch of unnecessary function calls and pointers and other forms of indirection that wouldn't be present in hand-written C. But for a more extreme example, you could also compile an interpreter or VM to C, and it would still be much slower than the equivalent hand-written C code. This is why "as fast as C" typically refers to normal, hand-written C code—even though there is no formal definition for what "normal C" looks like, it's still a useful description.