My iPhone 5S runs just fine...
... because I'm still running iOS v9. Yes it's annoying to have to dismiss the weekly "Do you want to upgrade?" and "Are you sure you don't?" (paraphrasing) messages, but by not upgrading the iOS I've skipped this very predictable problem entirely.
Sadly I wasn't so clever with my iPad which despite being blisteringly fast when I first got it, now struggles to notice that I'm even typing, forcing me to wait every 3rd word for it to catch up.
I realise that from a security point of view, not upgrading to the latest iOS is not ideal, but I don't do any internet banking on my phone so the risk is minimal.