Why does IO.puts print text so slow?

catalan.ex

defmodule Catalan do
  def main(argv) do
    List.first(argv)
    |> Integer.parse
    |> (&elem(&1, 0)).()
    |> parents
  end

  def parents(n), do: _parens("", n, n)

  def _parens(prefix, left, right) do
    unless left == 0 or right == 0 do
      IO.puts prefix
    end
    if left > 0 do
      _parens(prefix <> "(", left - 1, right)
    end
    if right > left do
      _parens(prefix <> ")", left, right - 1)
    end
  end
end