(* 2.257s *)
let () =
  let a = ref "asd" in
  let b = "qwe" in
  for i = 0 to 100000 do
    a := String.concat "" [!a; b]
  done

(* 2.002s, without conversion to string: 0.002s *)
let () =
  let buffer = Buffer.create 100 in
  Buffer.add_string buffer "asd";
  for i = 0 to 100000 do
    Buffer.add_string buffer "qwe";
    ignore (Buffer.contents buffer);
  done;