OCaml doesn't monomorphize functions. Instead references to every type are the same size (either a tagged int or a pointer). This is a sweet spot for OCaml, but doesn't really work for a language that doesn't allocate everything on the heap.
Indeed. Ocaml is GC'd, and that makes the implementation different. However, the question was about compile times, and i dare to say Ocaml is one if the fastest ones out there, even tho it has an rich and expressive typesystem. The conclusion then needs to be that the type system expressiveness (complexity?) does not alone make for slow compile times.