I'm sorry to say that, but I believe that you results are flawed...

If we look at the code of to_array_1 and to_array_5, there is no possibility that the former was faster... if nothing else, it has an additional if jump each and every loop. I simply couldn't believe your results.

Upon inspecting your code with Toploop, I found out some flaws...

let h () =
let h = Hashtbl.create 100000 in
    for i = 0 to 99999 do            (* <<< not Hashtbl.length h, as it returns 0 for ampty hashtable *)
      Hashtbl.add h (Random.int max_int) (Random.int max_int);
    done;
    h

let to_array_1 t =
let dummy = Array.init 0 (fun _ -> raise Not_found) in
    fst
      (Hashtbl.fold

         (fun k v (a, i) ->
            if i = 0 then
              let a = Array.make (Hashtbl.length t) (k, v) in
                (a, 1)           (* <<<<< Not 0, as it causes no progress *)
            else (a.(i) <- (k, v); (a, i + 1)))
         t (dummy, 0))

I also corrected my implementation:

let mgc = Obj.magic 0      <<< So that the function is executed only once.
let to_array_5 t =
let a = Array.make (Hashtbl.length t) mgc in
   ignore
     (Hashtbl.fold (fun k v i -> a.(i) <- (k, v); i + 1) t 0) ;
     a

I tried to do some benchmarking, but I do not have much time... anyhow, my implementation is faster as far as I tested it.

Believe in your dreams!