Avoid let and before blocks in your test when you don't really have to use them

simple_benchmark.rb

# user.rb file

class User
  attr_reader :admin

  def initialize(admin: false)
    @admin = admin
  end
end

# user_service.rb file

class UserService
  def initialize(user:)
    @user = user
  end

  def name
    if user.admin
      "ADMIN username"
    else
      "username"
    end
  end

  private
  attr_accessor :user
end

# Benchmarks using https://github.com/SamSaffron/memory_profiler gem

require './user'
require './user_service'
require 'memory_profiler'

report = MemoryProfiler.report do
  describe UserService do
    describe '#name' do
      it 'returns username with admin prefix when user is admin' do
        user = User.new(admin: true)

        user_service = UserService.new(user: user)

        expect(user_service.name).to eq('ADMIN username')
      end

      it 'returns username' do
        user = User.new(admin: false)

        user_service = UserService.new(user: user)

        expect(user_service.name).to eq('username')
      end
    end
    # describe '#name' do
    #   let(:user_service) { UserService.new(user: user) }
    #   let(:user) { User.new }

    #   before do
    #     allow(user).to receive(:admin).and_return(admin_user)
    #   end

    #   context 'when user is admin' do
    #     let(:admin_user) { true }

    #     it 'returns username with admin prefix' do
    #       expect(user_service.name).to eq('ADMIN username')
    #     end
    #   end

    #   context 'when user is not admin' do
    #     let(:admin_user) { false }

    #     it 'returns username without admin prefix' do
    #       expect(user_service.name).to eq('username')
    #     end
    #   end
    # end
  end
end

report.pretty_print

Version with let and before:

Total allocated: 1970221 bytes (20907 objects)
Total retained:  285854 bytes (2434 objects)

Version without let and before:

Total allocated: 1950093 bytes (20722 objects) (1,02% less)
Total retained:  274904 bytes (2367 objects) (3,83% less)