For those that are having issues getting it to work, initiate the download, change the date to before Feb 2014, open the app, then change the date back to auto. You can't leave the date/time different from current because nothing will download and it causes a lot of syncing problems (DropBox won't sync, etc).
It took a while for it to download a game to mine on a 15Mbps connection. Probably 1-2 minutes. It would be nice to have a progress indicator so you know it's not hanging.
As for the speed, it's dropping frames enough on my iPhone 5 to notice and the sound slows down. It's nothing that keeps the game from being played though and makes Mario Kart easier actually but not nearly as fast paced. I'd imagine its smooth on the 5S or iPad Air as long as it's optimized for the A7.
My has no sound at all, but it playing at about 65fps on my ipad air.
To be honest, it's pretty crazy that I can play a high quality NDS game on my iPhone.
Oh and DropBox sync support is awesome. I had to get my previous iP5 replaced and after I reinstalled GBA4iOS and turned on DropBox sync all my saves were seen and the games played exactly where I left off. Super convenient and makes using emulators on mobile devices worth it. Before that it was too much of a hassle to have to start the games all over again.
The only downside to GBA4iOS and NDS4iOS is that they still use the old d-pad's which can be frustrating on a touchscreen as they don't move around as your thumb tends to. A floating joystick ala Secret of Mana (App Store version) would be much, much better.