This damn game was amazing from start to finish, highly recommend it. Music sounds kind of lame, until you plug headphones in, then it's amazingly atmospheric and nuanced.
Gameplay was smooth as heck, some slowdown, but not too bad.
Even more slowdown on a stock 3DS I imagine, since disabling the L2 Cache on standard CPU clock resulted in quite a lot of slowdown in some instances. Using both L2 and New 3DS CPU clock resulted in a completely locked 30, but some very minor graphical artifacts, like lines across the screen at random, so I imagine that's why it uses standard clocks with L2 Cache enabled on New 3DS units.
Edit: Basically, playing on a New 3DS helps, even if you run standard firmware.