It's complex [1], OCaml does have a unboxed representation of floats in arrays [2] and records (provided all fields are floats), but elsewhere they are indeed boxed.
Good point. Polymorphic code that doesn't reduce to an unboxed float representation can be slower because of the boxing, but I don't think such code is very common exactly for this reason. I wonder if OCaml developers have tried NaN-boxing to see how it would impact performance.
[1] https://discuss.ocaml.org/t/optimizing-small-vector-operatio... [2] https://v2.ocaml.org/api/Float.Array.html