I think that is an irrational fear, but I also understand. (and I am on iOS 12 on my old 5s, the speed is not an issue for me. In fact, in my experience, and on my iP 5s, the response was slower on iOS 10 and 11, and seems "snappier" for most things on iOS 12. It's too bad that you can't try out iOS 12, then return to 9 if the performance is not acceptable for you. Or, you may have a friend who still has a 6s, and is now running iOS 12, so you could better understand what the upgrade may mean for response of the system.
Did you find the list of Apps that you were asking about? That has to be in your App store app somewhere. Just click around, and look for something that shows your AppleID. That would be where you will find your purchased apps.