Alternate body text:

I updated my phone and the new features are using more processing power than the previous OS did.

I have used iPhones since 2008, and this is a load of rubbish. There's a reason iPhones keep their resale value. They historically got updates for longer than most Android phones, and they do not get "bricked", literally or figuratively.

So you just installed the latest iOS update? Ever thought that caches might be cleared and it's getting more load after the reboot, or stuff getting re-indexed for a while? Or it's some bug you chanced upon, perhaps based on the iOS update and its interaction with some third party app you use a lot, and it'll get fixed if so?

As opposed to some mere point update with a few fixes and very marginal feature updates deliberately tanking your device or introducing anything major drain backend...