Not necessarily. Code in a language based on C can potentially be faster than it by having a deeper understanding of the code, and doing sophisticated optimizations C could not confidently apply. In Haskell, for instance, it often transforms multiple transforming passes over a list into one pass over the list with the three transformation operations composed (that is, map x (map y (map z l))) to map (x . y . z) l). I'm sure there are much, much better examples...

This isn't 100% true, because in theory you could have written out the ideally optimized version of machine language for what you're trying to do, or (perhaps) a direct transliteration thereof in C. In practice, this isn't realistically possible, except in tiny pieces.

See also: Proebsting's Law (http://research.microsoft.com/~toddpro/papers/law.htm)