I have an iPad Air 2 64GB (which was introduced along with the iPhone 6 or 5S), and I've always kept it updated. I've never faced a slowdown as such, though there are some stutters once or twice a day. Nothing serious at all, and I'm amazed it's still receiving updates more than 5 years after launch. Still works incredibly well and even multitasks smoothly (Safari and a PDF reader side by side, with music playing).
Id recommend you should update your iOS device based on the above, plus a friend who owns a 6S+ is sitll happy with it, and refuses to upgrade because it's the last iPhone to have a 3.5mm jack.