I started this thread and I"m still having problems. The significant increase in battery drain is well-documented. I was already in need of a battery replacement before the upgrade and the upgrade made the situation much worse. I proceeded with replacing my battery and that definitely helped, but it didn’t cure the problems I had with apps loading very slowly. Virtually all non-Apple apps get stuck on the splash screen for several seconds, and many apps crash after about twenty seconds, but then load normally after that. The one exception is the Amazon app, which crashes every time. I’ve tried deleting and reinstalling it, several times, with no change.The phone freezes up for several seconds whenever loading content that used to load instantaneously. It’s enough to make me downgrade if I can’t find another solution.
Again, I have a 256GB iPhone SE that has served me well of over 3 years, and see no need to upgrade for now. My next step will be to reset my iPhone to factory specs, but that’s not undertaken lightly as it means setting up a number of things from scratch, including every app that uses Touch ID. That’s better than buying a new phone, though.
One more thing on a related note, the battery drain on my Apple Series 8 watch is much worse after the upgrade to Watch OS 10. I used to get nearly a day and a half of use out of a charge, and now I have to recharge more than once a day.