For 3DS users, it's been mentioned before in the thread I believe, but since there are a lot of new folks on...
The framerate on the 3ds is much better with the 3d turned off. But not via the slider, for it to affect the framerate you have to quit the game and go to the main screen's options and disable the 3d display. Voila, instant ~40-45 fps.
For me personally, I'll take a smoother framerate any day. The 3d is neat for a bit, but not worth the frame hit. It's also tough for me to maintain the right aspect to keep the 3d in effect when I'm having to swivel and tap the L button and do all that jazz.