Bit confused there, the phone is reindexing itself to work efficiently with iOS 11?
I don’t spend my days on forums but never heard that one before. If Apple had known this was going to happen, they could have prepared owners for it or fixed the Update before it was released.
The only explanation l would give is that the iOS 11 has some problems but lots of great points and owners love finding out how to use them and are draining the batteries somewhat faster.
Maybe now they are getting used to iOS 11