2D games runs on old3ds?
Well, with proper roms I have been able to play:
Symphony of the Night (33to60 fps deppending on the rom, pbp format, no music but with sound effects still there, could be further improved if I found the way to turn off some of the background layers)
Marvel vs Capcom (45 fps no music, still with sound effects)
Guilty Gear
Pocket Fighter
there might be some more games I'm forgetting right now. Also we could see some improvement if someone implemented the plugin that the latest pcsx_rearmed_libretro uses in the version from 1.3.6 (latest version that I found runs in Old3ds)