very dumb is even function benchmarked tco vs. no tco

number.ex

defmodule Number do
  def is_even?(0), do: true
  def is_even?(n) do
    !is_even?(n - 1)
  end

  def is_even_tco?(n, acc \\ true)
  def is_even_tco?(0, acc), do: acc
  def is_even_tco?(n, acc) do
    is_even_tco?(n - 1, !acc)
  end
end

number_is_even_bench.exs

number = 10_000_000

Benchee.run %{
  "is_even?" => fn -> Number.is_even?(number) end,
  "is_even_tco?" => fn -> Number.is_even_tco?(number) end,
}

results

tobi@happy ~/github/elixir_playground $ mix run bench/is_even.exs 
Benchmarking is_even?...
Benchmarking is_even_tco?...

Name                                    ips        average    deviation         median
is_even?                              10.26     97449.21μs     (±0.50%)     97263.00μs
is_even_tco?                           9.39    106484.48μs     (±0.09%)    106459.50μs

Comparison: 
is_even?                              10.26
is_even_tco?                           9.39 - 1.09x slower