[achou]

Sure, I think what I noticed was that even idiomatic OCaml code was relatively fast, maybe 2-3x slower than C, but plenty fast enough. Whereas I was under the impression that idiomatic Haskell was far more likely to have unexpected and harder to fix performance issues (e.g. requiring more of an architectural rewrite) because of its lazy evaluation model.

[kccqzy]

The problems brought about by the lazy evaluation (laziness when you don't want it) do not require architectural rewrites. It's mostly just profiling while setting a very low stack size limit, and then discovering which part of your code triggers a stack overflow. It can be solved by adding the right amount of seq (sequences evaluation), or ! (strict patterns). Maybe you'll also change a few incorrect uses of foldl into foldr. Even if you need to change from lazy Map to strict Map the change isn't disruptive; it's just changing some import and done; all the functions work. No architectural changes needed.