I decide to simply add recommended devices in the app description..
My slowest device for testing is an iPod Touch 4th Gen so that's the slowest device that I KNOW it runs well on.
I imagine it will run well on iPod Touch 3rd Gen devices (not including the 8GB model which only has 128mb system ram), but since I don't own one for testing I'm only recommending what I know works well. Hopefully those with earlier generation devices than my recommendations won't complain if it runs a bit slower.
My device recommendations were as follows:
iPod Touch (4th Gen or later), iPhone 4 (or later), iPad (Any)