benchmark.exs

defmodule Tester do
  def update!(map) do
    try do
      {:ok, Map.update!(map, :blah, fn v -> v end)}
    rescue
      e in KeyError -> {:error, :no_key}
    end
  end

  def update(map) do
    {:ok, Map.update(map, :blah, %{a: 1}, fn v -> v end)}
  end

  def check_and_update!(map) do
    if Map.has_key?(map, :blah) do
      {:ok, Map.update!(map, :blah, fn v -> v end)}
    else
      {:error, :no_key}
    end
  end
end

with_key = %{:blah => 1}
without_key = %{}

Benchee.run(%{
  "check_and_update! - key exists" => fn -> Tester.check_and_update!(with_key) end,
  "update! - key exists" => fn -> Tester.update!(with_key) end,
  "update - key exists" => fn -> Tester.update(with_key) end,
  "check_and_update! - key does not exist" => fn -> Tester.check_and_update!(without_key) end,
  "update! - key does not exist" => fn -> Tester.update!(without_key) end,
  "update - key does not exist" => fn -> Tester.update(without_key) end,
})

results.txt

Name                                             ips        average  deviation         median         99th %
check_and_update! - key does not exist       31.67 M       31.57 ns  ±1173.96%           0 ns         990 ns
update - key does not exist                  10.53 M       94.96 ns ±30972.53%           0 ns         990 ns
update - key exists                          10.02 M       99.81 ns ±33800.81%           0 ns         990 ns
check_and_update! - key exists                9.98 M      100.18 ns ±26534.11%           0 ns         990 ns
update! - key exists                          6.98 M      143.30 ns ±23903.76%           0 ns         990 ns
update! - key does not exist                  1.60 M      625.12 ns  ±1441.31%         990 ns        1990 ns

Comparison:
check_and_update! - key does not exist       31.67 M
update - key does not exist                  10.53 M - 3.01x slower +63.38 ns
update - key exists                          10.02 M - 3.16x slower +68.24 ns
check_and_update! - key exists                9.98 M - 3.17x slower +68.61 ns
update! - key exists                          6.98 M - 4.54x slower +111.73 ns
update! - key does not exist                  1.60 M - 19.80x slower +593.55 ns