Generally, yes. An experienced Haskell guru can optimize the heck out of everything (see [1], for example), sometimes even beating C. But still there is a lot

Generally, yes. An experienced Haskell guru can optimize the heck out of everything (see [1], for example), sometimes even beating C. But still there is a lot of traps, a lot of situations where high-level nature of the language makes writing an efficient implementation harder and obscures that (unless you're an expert in GHC internals). For example, darcs is known for accidental problems on large projects (though I can't corroborate this from my experience), maybe of algorithmic nature. From my experience: choosing a right library matters much. Choosing one numeric library from cabal instead of another had drastically boosted performance of a number-crushing task for a crypto course. A simple program using HXT (xml processing library with beautiful mind-boggling API based on arrows) has exploded on a large input. Also, I am interested pretty much what's the largest site written in Yesod/Happstack/Snap and how well they behave there (these frameworks' benchmarks are mostly good and promising). [1] - http://benchmarksgame.alioth.debian.org/u32/haskell.php http://benchmarksgame.alioth.debian.org/u32/haskell.php