I just purchased the game today on the eShop and played it first in portable mode. I couldn't believe how bad the frame rate got within the first ten minutes of the game when you do battle. The more enemies that appeared on screen the worse it got.
Docked mode however seems to run pretty stable for the most part with a few random hiccups here and there. Just an FYI since the big sites don't seem to be reporting much on the game.
Games are fun.