The fact that it uses C as a compilation target doesn't have much to do with whether it's as fast as C in practice. It's easy to imagine a compiler that generates C but generates terrible C that runs really slowly. (E.g., imagine it goes via some sort of stack-machine intermediate representation, and variables in the source language turn into things like stack[25] in the compiled code.)
Or consider: Every native-compiled language ends up as machine code, which is equivalent to assembly language, but for most (perhaps all) it would be grossly misleading to say "as fast as assembler".
Or consider: Every native-compiled language ends up as machine code, which is equivalent to assembly language, but for most (perhaps all) it would be grossly misleading to say "as fast as assembler".