Hmmm really? The Mini 2 seems to perform pretty damn well for me on iOS8. It's my iPad Mini 1 that is nigh unuseable with iOS8. I don't even know if I'll be able to unload the first mini. That thing is SOOOOOOOOOOOOOOOOOOOOOOOO slow.
It's not bad, but I upgraded from iphone 4S to 6 and iOS8 is so much smoother and nicer on the iphone 6 than my retina mini