b.rb

require 'benchmark'

class Foo
  attr_reader :bar

  def initialize
    @bar = 'bar'
  end

  def with_local_var
    bar = self.bar
    if bar == 'bar'
      bar
    end
  end

  def without_local_var
    if bar == 'bar'
      bar
    end
  end
end

f = Foo.new
n = 500000

Benchmark.bm do |x|
  x.report('with_local_var')    { f.with_local_var }
  x.report('without_local_var') { f.without_local_var }
end

gistfile1.txt

                   user       system     total    real
with_local_var     0.000000   0.000000   0.000000 (0.000005)
without_local_var  0.000000   0.000000   0.000000 (0.000003)