I Know exactly what you mean. I had this problem with a new iPad Air 2. after upgrading ios 9 to another 9.1, my battery performance went from good to pretty bad. Nothing on my iPad had changed other than the iOS update. It seems to be an obscure bug, i have reported it and I'm even on public Beta now to try to get more feedback on the problem. Its sad, but unless someone witnesses the problem on their device, they tend to believe that it must be user error. All I can say is that I am seeing more and more reports on the problem manifesting itself, so hopefully apple with take notice and make a serious look into the phenomena. it happens after an iOS update, but it's not a blanket occurance, so it's hard to replicate.
i Would definitely send feedback about the issue to Apple. That should be made aware of just how more common this issue is becoming.
It seems that the common 'fix' given is a link to an article on how to conserve battery power, but that goes nowhere near solving the issue and just changes how you use your device when there was no problem beforehand.