Okay, I just spent the last couple hours getting this to work and testing. I'm on an OG 3DS.
NES and Game Boy games are 100% awesome. Great quality - way better than even on my Galaxy S5. I don't know why, it's just awesome.
I tried Super Mario World and Mega Man X - both ran at an unplayable 26 fps in the recommended SNES emulator. In the other SNES emulator, it was 12fps. So.... yeah. It's unplayable. I haven't tried BlargSNES yet, though. Might need to do that.
I tried Pokemon Fire Red on mVBA. It ran at 11 fps. Also unplayable.
So, unless you have a N3DS, you won't be playing any SNES or VBA games any time soon. However, the GB and NES emulators are top notch.
I was reading in their forums, and it looks like the drastic difference is caused by how much CPU the software is allowed to access. On a New3DS, it can access 80% of the CPU. On an OG 3DS, it can only access 30%.
How's the aspect ratio? Is it 8:7 or 4:3?
It's 4:3, but you can force custom aspect ratios.