The advance that purely functional languages offers is the availing of hitherto unknown, extremely powerful optimizations. Small changes at the level of Haskell source can lead to massive, often unexpected changes in runtime performance. Newcomers often find the "discontinuous" experience disconcerting (and that's not even counting encounters with the type checker).

The good news is that once Haskell source gets blender'd into Core, performance becomes far more predictable. Hence, those who care about every last drop of time/space usage read Core and put work into making it more readable.