Hmm. Well, I just tried without retina display enabled, and the frame rate improved to 40-60fps. Still not as strong as my Lumia 920's performance, but then that phone is one and a half years newer than the iPad 3, so maybe that's to be expected.
I think all iOS releases from this point on HAVE to support retina display though, so I guess this means I need to optimize the game further for mobile and tablet devices. Thanks for all the feedback about the importance of downscaling though! It's been very useful