attr_reader vs instance var

benchmark.rb

require 'benchmark/ips'

class Foo
  attr_reader :test

  def initialize(test)
    @test = test
  end

  def read
    1000.times { test }
  end
end

class Bar
  def initialize(test)
    @test = test
  end

  def read
    1000.times { @test }
  end
end

Benchmark.ips do |x|
  x.config(time: 10)
  foo = Foo.new('lalala')
  bar = Bar.new('lalala')
  x.report('attr') { foo.read }
  x.report('instance') { bar.read }
  x.compare!
end

Calculating -------------------------------------
                attr     2.224k i/100ms
            instance     2.680k i/100ms
-------------------------------------------------
                attr     23.315k (± 0.4%) i/s -    233.520k
            instance     27.291k (± 0.5%) i/s -    273.360k

Comparison:
            instance:    27291.5 i/s
                attr:    23314.7 i/s - 1.17x slower