You could just look at the code instead of making up a strawman to try to dismiss the only evidence presented. Poor evidence > no evidence.
The fact that this code is fast tells you nothing about whether idiomatic Haskell is fast. wrong evidence < no evidence.
The question was about difficulty, not a subjective idea of what idiomatic code looks like. Are you seriously claiming that the average haskell example there is more difficult to write than the average ocaml example? If so, just say that. Trying to redirect the discussion back to your subjective preferences is not productive.
And `inlinePerformIO` is much more unsafe, because it requires that you do no memory allocation inside it. This property is specific to the Haskell implementation, and is certainly not easy to guarantee.