These kinds of situations are hard to find a fix for due to so many variables involved.
Aside from the obvious (background refresh, apps that were notorious for battery drain -> facebook), the only thing you can do is either hard reset the phone and restore it, or take it to the apple store and have them diagnose it.
If you do take it to the apple store, don't take no for an answer. You paid for the phone and warranty that comes with it, don't let them give you the run around and settle for a crap phone/experience!
On a side note, I upgraded to iOS11 yesterday afternoon with my 7+, the first 30 minutes were laggy but afterward it's been smooth sailing. I am meticulous with notifications, so only the essential apps like phone, messages and mail all prompt a notification but everything else is silenced. The battery performance for today has been the same as with iOS10, so it only took my phone about 30 minutes to get everything roughed out, hope that anecdote helps!