assign_bench.rb

class A
  def normal
    @a = nil
    @a ||= 1
  end

  def other_
    @b = nil
    @b = 1 if !@b
  end
end

require 'benchmark'
i = A.new

puts Benchmark.measure{1000000.times{i.normal}} # => 0.190000   0.000000   0.190000 (  0.194127)
puts Benchmark.measure{1000000.times{i.other_}} # => 0.150000   0.000000   0.150000 (  0.164706)