I currently am using a class-4 and it seems to work okay as far as DS games go, but my GBA games have noticeable slow down; all of them. Would a class-10 help as well as making everything else run better?

 

Hey Azureth, have you tried increasing the CPU frequency under the "Others" setting category in the GBA emulator? A lot of roms will work much better at a setting of 3 or 4, at the cost of battery life.
Unfortunately getting a high class SD card won't affect slowdown in the games: if anything it'll just improve initial loading time. However a class 10 SD is a lot faster when copying files to the SD card from your computer, which is why I got one.