One concern about upgrading older iPhones to a new iOS is it will be slowed down by the new operating system. Each recent version of iOS has more bells and whistles, so the logical conclusion is to assume it will be slow on older devices. Apparently, that is not the case with the recently released iOS 14. This article describes tests done on older iPhones using iOS 14. It does not look too bad at all. I have an iPhone 11, but if I had an older iPhone, I would not hesitate to update to iOS 14 if my phone qualified for the update.