bench.rb

require "benchmark"

class A
  def initialize(atts)
    atts.each do |k,v|
      instance_variable_set(:"@#{k}", v)
    end
  end
end

class B
  attr_accessor :foo, :bar, :baz

  def initialize(atts)
    atts.each do |k,v|
      send(:"#{k}=", v)
    end
  end
end

class C
  def initialize(atts)
    @foo = atts[:foo]
    @bar = atts[:bar]
    @baz = atts[:baz]
  end
end

Benchmark.bmbm do |x|
  x.report "iv" do
    1000.times do
      A.new(foo: "foo", bar: "bar", baz: "baz")
    end
  end

  x.report "send" do
    1000.times do
      B.new(foo: "foo", bar: "bar", baz: "baz")
    end
  end

  x.report "straight" do
    1000.times do
      C.new(foo: "foo", bar: "bar", baz: "baz")
    end
  end
end

results.txt

Rehearsal --------------------------------------------
iv         0.000000   0.000000   0.000000 (  0.004341)
send       0.010000   0.000000   0.010000 (  0.004375)
straight   0.000000   0.000000   0.000000 (  0.001497)
----------------------------------- total: 0.010000sec

               user     system      total        real
iv         0.000000   0.000000   0.000000 (  0.003495)
send       0.010000   0.000000   0.010000 (  0.003770)
straight   0.000000   0.000000   0.000000 (  0.001198)