I suppose in FP/Haskell you would structure your code differently, so that it wouldn't have side effects when mapping arrays. That way you could even parallelize it trivially.

High level languages produce a lot less code than than less optimized implementations. Difficulties with reasoning about execution/performance is the penalty you potentially have to pay.

In my experience it indeed may be hard to understand performance with Haskell. That said, it comes with excellent tooling to overcome it in most of the cases.