I won't comment on Haskell's speed here, but function composition can't possibly be making Haskell less faster.

The reason is simple: the compiler inlines the composition operator in the vast majority of (probably all?) cases, since the Haskell standard library source code marks it with an INLINE pragma:

   -- | Function composition.
   {-# INLINE (.) #-}
   (.)    :: (b -> c) -> (a -> b) -> a -> c
   (.) f g = \x -> f (g x)

http://hackage.haskell.org/package/base-4.10.0.0/docs/src/GH....