Benchmark.ips do |x|
  x.report("attr_reader") do
    class Foo
      def initialize(bar)
        @bar = bar
      end

      def baz
        bar.length
      end

      private

      attr_reader :bar
    end

    Foo.new("asdf").baz
  end

  x.report("ivar") do
    class Foo2
      def initialize(bar)
        @bar = bar
      end

      def baz
        @bar.length
      end
    end

    Foo2.new("asdf").baz
  end

  x.compare!
end

#=>Calculating -------------------------------------
#         attr_reader     1.901k i/100ms
#                ivar     2.060k i/100ms
#-------------------------------------------------
#         attr_reader    262.574k (±23.5%) i/s -    851.648k
#                ivar    404.494k (±24.4%) i/s -      1.234M
#
#Comparison:
#                ivar:   404494.5 i/s
#         attr_reader:   262574.3 i/s - 1.54x slower