From my experience: * The vast majority of code doesn't need to be fast, and it's trivial to write code that's within a factor of 5 of optimized C. I can writ

From my experience: * The vast majority of code doesn't need to be fast, and it's trivial to write code that's within a factor of 5 of optimized C. I can write Haskell at least 10x faster than I can write optimized C for the same problem. * If you want code that's within a factor of 2 of C then you'll need to understand quite a bit about the internal details of the compiler, but your code won't get too ugly. I'd say it's about the same amount of work in both C and Haskell to get this level of performance. * If you want code that's just as fast as optimized C then you'll need great understanding of both the compiler and the underlying hardware. At this point, you can expect your code will get hideously ugly. Also your code will break a lot when GHC versions change. I can write optimized C about 10x faster than I can write optimized Haskell. Of course every problem is different, and this is only a rough average of my experience.