Okay, so you wish to know what the DSTWO can do? I can help you there my friend as I just recently got it. Though the emulators I have may differ from what you get as yours may not have these options. If you wish, you can message me about what emulators I got on mine in case you can't find any yourself.
It can run GBA games at an average speed, DS or 3DS; though you can change this in the menu to set it to 0 frame skips (though it might drain your power more; I'm not sure). It might change it back to at least 1 or 2 if it knows it cannot handle it (my 3DS could so it's all good). A DS could handle GBA games as it had the GBA slot (so then again, why are you using DSTWO? Maybe for the other games I suppose; or maybe you just have a 3DS and we all know that doesn't have the GBA slot so that sucks), though sometimes, maybe because of all the other roms and emulators on the cartridge, that could cause it to have to slow down in order to keep power and gameplay at hand. So it is decent with GBA games I suppose.
It can run SNES games at average speed also, though you can change the processing power to 4 (yeah, processing power - not frame skips - as it is different with the SNES emulator); though this, in return, will drain your DS battery significantly; DS or 3DS. Because running the game at full speed takes power of course! That isn't a problem for a computer though; so it just seemed more easier at the time. I recommend keeping the processing power at at 2.
And finally, the DS games themselves; it can run those at a great speed. Though I don't remember how the games ran on an original DS, it seems to emulate them pretty well. There are no problems with it, and it can support wifi as well! Just be sure to flip the wireless switch before booting up the cartridge as it cannot be turned on once you're in there (though it can be turned off - which is weird - as I cannot turn it on but off). Though as a source told me, it does not support wifi for the new Pokémon Black & White games (as the cartridge itself contained a separate processor for this; I do not know why).
Oh, I almost forgot. It can support GB games too; both Original and Color. So you could play like Pokémon Red, Blue or perhaps another GB game of your choice. It runs at a great speed and you will most likely not encounter any problems with it.
That is all I have to share with you today. If you have any further questions about DSTWO you can send me a message if you like; I will be more than happy to answer it when I find the time. Though I have a lot of time on my hands, so I might be able to answer it quickly too if I can.