This commit might interest you: 3c3055d

9.0.2.0+ will contain this, but it is currently more limited than it needs to be (as outlined in the commit message). So with your sample snippet above we now run that method a couple times faster than 1.7, but many other define_methods will probably show the slowdown you have reported.

For record here is a benchmark I used for the commit I mentioned above:

require 'benchmark/ips'

class Foo
  b = 1
  define_method(:fooced) do |a|
    b
    a
  end

  define_method(:foo) do |a|
    a
  end

  def bar(a)
    a
  end
end

foo = Foo.new


Benchmark.ips do |x|
  x.report "define_method w/ capture" do
    foo.fooced(1)
    foo.fooced(1)
    foo.fooced(1)
    foo.fooced(1)
    foo.fooced(1)
    foo.fooced(1)
    foo.fooced(1)
    foo.fooced(1)
    foo.fooced(1)
    foo.fooced(1)
  end

  x.report "def" do
    foo.bar(1)
    foo.bar(1)
    foo.bar(1)
    foo.bar(1)
    foo.bar(1)
    foo.bar(1)
    foo.bar(1)
    foo.bar(1)
    foo.bar(1)
    foo.bar(1)
  end

  x.report "define_method" do
    foo.foo(1)
    foo.foo(1)
    foo.foo(1)
    foo.foo(1)
    foo.foo(1)
    foo.foo(1)
    foo.foo(1)
    foo.foo(1)
    foo.foo(1)
    foo.foo(1)
  end


end

@enebo Thanks for point out that commit, I saw it over the weekend and got super excited. The case that led to creating this issue was a block referencing parent scope which would not take advantage of your optimization. I'm really looking forward to seeing what comes next though :).

My numbers on @enebo's bench actually seem to show that define_method with capture is currently faster on 9k than on 1.7 (and of course without capture it's way faster now):

[] ~/projects/jruby $ jruby bench_define_method.rb 
Calculating -------------------------------------
define_method w/ capture
                        53.266k i/100ms
                 def   105.543k i/100ms
       define_method    84.974k i/100ms
-------------------------------------------------
define_method w/ capture
                          1.093M (± 6.9%) i/s -      5.486M
                 def      3.751M (± 8.5%) i/s -     18.576M
       define_method      2.864M (± 7.6%) i/s -     14.276M

[] ~/projects/jruby $ rvm jruby-1.7.21 do ruby bench_define_method.rb 
Calculating -------------------------------------
define_method w/ capture
                        49.267k i/100ms
                 def   150.373k i/100ms
       define_method    57.282k i/100ms
-------------------------------------------------
define_method w/ capture
                        821.773k (± 5.6%) i/s -      4.138M
                 def      5.311M (± 8.7%) i/s -     26.315M
       define_method    825.927k (± 4.9%) i/s -      4.124M

Of larger worry is the performance of a plain def here, but that's not related to this bug either.

@headius @enebo The benchmark-ips gem is awesome! I'm seeing similar results to you guys, meaning define_method is much faster, but def is slower (different issue, like you said, @headius). Thanks for looking into this guys. I'll keep profiling and let you know if I find anything :)