J3RN

sample = Enum.map(1..1000, &{&1, &1}) |> Map.new()

Benchee.run(%{
  "maps:map" => fn -> :maps.map(fn _k, v -> v * 2 end, sample) end,
  "Map.new" => fn -> Map.new(sample, fn {k, v} -> {k, v * 2} end) end,
  "for" => fn -> for {k, v} <- sample, into: %{}, do: {k, v * 2} end
})

J3RN

****** Process <0.91.0>    -- 100.00 % of profiled time *** 
FUNCTION                              CALLS        %   TIME  [uS / CALLS]
--------                              -----  -------   ----  [----------]
erl_eval:exprs/5                          1     0.00      0  [      0.00]
erl_eval:ret_expr/3                       2     0.00      0  [      0.00]
erl_eval:eval_fun/2                       1     0.00      0  [      0.00]
erl_eval:expr_list/4                      1     0.00      0  [      0.00]
erl_eval:guard/4                          1     0.00      0  [      0.00]
erl_eval:guard0/4                         1     0.00      0  [      0.00]
erl_eval:match_list/4                     1     0.00      0  [      0.00]