[bluGill]

Only in some very specific situations, which amount to an arbitrarily smart compiler and arbitrarily dumb C code. Haskell does provide more opportunities for the code to not do something you tell it do because it can prove you don't use the result, but it is a dumb programmer who is writing code to do those unneeded calculations, and thus a good programmer would eliminate them from C.

A lot of effort has gone into optimizing C compilers, Haskell is easier to optimize in theory, but in practice it isn't enough easier as to overcome the massively larger amount of effort put in C.

In the best case for both the code will be roughly the same speed. However the best case for both can look very different and any comparison is suspect as it is likely that whoever wrote the code wasn't as good at one as the other and so wrote bad code.