The "oh no it's slow, and you can't reason about performance" FUD is mostly directed at Haskell's lazy evaluation, but people like to throw it vaguely in the direction of "FP" in general. Most of the performance problems you have in Haskell (as in this recent submission: https://news.ycombinator.com/item?id=21266201) are not problems you will have in OCaml.

Yes, OCaml has garbage collection. It's a very efficient GC, and it is only ever called when you try to allocate something and the system determines that it's time for cleanup (https://ocaml.org/learn/tutorials/garbage_collection.html, though this might change if/when Multicore OCaml ever happens?). So if you write an innermost function that does arithmetic on stuff but never allocates data structures, you will not have GC problems because you will not have GC during that time, period.

Also, there are cases where destructive mutation of things is more efficient than making pure copies. OCaml allows you to do that, you don't need to fiddle with monads to simulate state.

There really isn't that much black magic there. Just don't believe everything that is said about "FP".