>I find it much easier

Ok, but your anecodes + my anecdotes = still just anecdotes. To support the claim that it is easier to write fast code in ocaml, we need to find evidence.

>Writing the equivalent Haskell code is much more work

I have not experienced anything like that, despite having much more experience with ocaml than with haskell. You are imposing limitations on yourself when using haskell and then saying haskell is imposing them. Every ocaml function is in IO, it just doesn't provide type information to tell you that. So doing the same thing in haskell is doing it in IO, which requires no special effort at all. Even if you choose to add a purity restriction on yourself, adding "runST $" does not seem like a lot of work to me.