Just to add more context: Bcrypt is slow by design. Password hashing needs to be slow to provide the security they give. Usually in tests one would dial down all the knobs, which allow it to be faster, or even replace hashing with a dummy.

One way would be to wrap it in a behaviour module

defmodule MyApp.Accounts.PasswordHash do
  @callback hash(String.t) :: String.t
  @callback verify(String.t, String.t) :: boolean

  impl_mod = Application.get_env(MyApp.Accounts.PasswordHash, :impl)
  
  defdelegate hash(value), to: impl_mod.hash(value)
  defdelegate verify(value, hash), do: impl_mod.verify(value, hash)
end

and have different implementations for test and dev/prod, where in the former one it would return a string

# this would probably be defined in test/support/
defmodule MyApp.Accounts.PasswordHashDummy do
  @behaviour MyApp.Accounts.PasswordHash
  def hash(value), do: "hashed_" <> value
  def verify(value, hash) do
    "hashed_" <> value == hash
  end
end

and in the latter – delegate to bcrypt

# defined in lib/
defmodule MyApp.Accounts.PasswordHashDefault do
  @behaviour MyApp.Accounts.PasswordHash
  defdelegate hash(value), do: Bcrypt.hash_pwd(value)
  defdelegate verify(value, hash), do: Bcrypt.verify_pass(value, hash)
end

and the appropriate implementation module would be set in env’s config

# config/config.exs
config MyApp.Accounts.PasswordHash, impl: MyApp.Accounts.PasswordHashDefault

# config/test.exs
config MyApp.Accounts.PasswordHash, impl: MyApp.Accounts.PasswordHashDummy

and the rest of the app would use MyApp.Accounts.PasswordHash.hash/1 and MyApp.Accounts.PasswordHash.verify/2.

Did you try reducing log_rounds in test env, as advised in step 3 of the installation guide? This is what I typically do in all of the projects, and tests are running smooth then. I never had to do this complex mocking of bcrypt.