I’m having the same situation on iPhone 7 iOS 14.4.2—many apps are behaving strangely and there is a general slowdown in the OS:
camera: opens with black camera view and has to be hard quit. Can switch between modes but still black view.
mail: long press preview of messages freezes the app, sometimes glitches out completely and layers of the UI get mixed together.
Facebook: won’t dismiss notifications and reacts/comments don’t appear until you go off and back on the posts. Frequently freezes completely and must be hard quit.
Messenger: keyboard won’t dismiss at all in chats, so gifs and stickers cannot be touched and messages won’t mark as read unless responded to. Videos play audio but show the first frame image only.
WhatsApp: won’t stay open for longer than 10 seconds, often crashes instantly on open.
Youtube: won’t auto-rotate videos.
my iPad Pro 10.5” has also been slow on iOS 14.4.x iterations too. Something feels very wrong and I am worried that, because these devices are older, that I will be left as my last iOS version on a buggy release. I am not sure if iPhone 7 will get iOS 15 but either way, to have a buggy final x.x.x is going to leave some users their last iOS update possible as a bug-ridden one. Perhaps iPhone 6S...