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")
      A.new(foo: "foo")
    end
  end

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

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

foo.txt

Rehearsal --------------------------------------------
iv         0.000000   0.000000   0.000000 (  0.002298)
send       0.000000   0.000000   0.000000 (  0.001946)
straight   0.000000   0.000000   0.000000 (  0.001161)
----------------------------------- total: 0.000000sec

               user     system      total        real
iv         0.000000   0.000000   0.000000 (  0.001688)
send       0.000000   0.000000   0.000000 (  0.001677)
straight   0.000000   0.000000   0.000000 (  0.000776)