On to iPhones. I have an iPhone 4S on iOS 9 too. It’s jailbroken because the sleep button doesn’t work so I needed a work around. It’s too slow for me though. Great if you don’t want to be able to use your phone much. Otherwise the wait to load apps and the limited or weird acting ram is annoying. Nonetheless when I want to try reducing my digital “addiction” I can get by using it. My friends find it too slow to even use for more than 2 min.
On the other hand, I also had a chance to use an iPhone 5 or 5S in the summer. It was perfectly usable. Fast enough and not that old of an OS (I think iOS 10) to be useless for downloading new apps.