The vast majority of iOS users who upgraded saw no performance hit at all and most saw improvements. . . so there was nothing to "warn" about. A minority of the millions of iPads, apparently yours among them, developed problems with the iOS 8.0 version. I had problems i documented on FR and documented to Apple, but no loss of battery time. Apple quickly released an 8.1 update that addressed most of those problems. 8.1 solved most of the issues I saw. Did you install it? 8.1.2 is intended to catch more.
My comment was limited to iPad 2 owners who upgraded. And Apples own message boards reflect hundreds of iPad 2 owners who had the same issues. One can only assume there were legions more who don't post on message boards. There are dozens of magazine and blog articles which acknowledge the issues and try to offer tips and work arounds to try and lessen these effects. Given that the only solution Apple offers is to buy a new tablet tells me all I need to know.