We have a 2 and 3 both on 9 and both are fine, bit slow at times with app loading, but nothing to get bothered about. Guess it's personal though as to what is ok or what is too slow.
I have been wondering about this as well. I upgraded to 8.0 and next day downgraded back to the last version of 7.
Unless I heard or see different, I will stay with that version of 7.