Adding the specific things that break in an update and produce exactly your symptom, all of which are fixable in minutes.
An app stuck in a retry loop. Something that lost its credentials in the update and is now retrying in the background forever. Shows up as one app with a lot of background activity and almost no screen time. Signing out and back in fixes it; this is by far the most common real cause.
Location permissions reset to always. Updates sometimes reprompt, people tap through, and an app that used to have location only while in use now has it constantly. Worth reviewing after any major update regardless.
A settings default that changed. Background refresh re-enabled for everything, or a display setting that moved. These are silent and they add up.
Something genuinely unfinished. Photo analysis in particular can take a very long time on a large library, and it is visible — there is usually an indicator in the photos app telling you it is still working. If it says it is still processing, you have your answer directly rather than by inference.
On the downgrade question: I would not. The window is short, the process wipes the device in practice, and the overwhelming majority of these resolve either by themselves or by finding the one app above. Downgrading to fix a battery problem you have not diagnosed is a large amount of risk for a guess.