I got that error message during the beta period.
I think they're trying to make Keychain more secure and it's causing a few bugs.
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Probably indexing. Wait a day or two after installing the update.
Lol,indexing.Like there is 0.1 inch 3000 rpm HDD inside iPhone.Dude,that is flash storage.Even if there is happening some kind of "indexing" it would be done in 5 min and wouldn't be draining battery the whole day. Only explanation of "better performance after 10.3 update" is increased usage of phone resources to achieve that goal which leads to increased consumption of power.