Newellj: Yes, look a few threads down -- I started a thread about it with specific examples. I own both an iPad Air 2 and Mini 4 on iOS 9.0.1, as well as iPod Touch 5 on iOS 7.
Others have corroborated my findings. There is something amiss, but I don't think it will bother you unless you play games. For people who are mainly reading and watching media, the Mini 4 is great. The screen is superb -- even slightly better than Air 2 (can go a little brighter). But gaming performance is poor in a number of games; Air 2 is much better.
Thanks for the link. Too bad it's not possible to run a Mini 4 on iOS 8.x, just for purposes of comparison (which is obviously completely impossible...).