Killer Instinct Arcade & Killer Instinct 2, plus Street Fighter Alpha 3, running on my iPhone 5 using Gridlee app - MAME4iOS Reloaded v 1.3 by David Valdeita (Seluco).

all games run better than what your are viewing, the display recorder used makes it look worse than it actually is.

Update: The App has been removed from Apple AppStore, But you can follow the tutorial below to get it back on your iPhone/iPod/iPad without the need of a jailbreak or appsync - http://www.youtube.com/watch?v=QkndJ0kg1rw