This is entirely unrelated to Swift. But to address our concern: your iPhone 6 Plus is the slowest iPhone still getting updates today. Sure, it should perform better, and the iOS 12 update later this year will help fix this issue, but Swift has nothing to do with making your phone slow.
I upgraded to the iOS 12 beta a week or so ago on my 6S+. I can say without any doubt that it is so much faster that I feel like I have a new phone in my hand. I honestly never thought an upgrade could make this much difference.
Not sure what apps you are talking about, but not many core apps have been rewritten in Swift.
Also, if Swift is so much better and is supposed to be safer to use why are apps much slower and buggier than I ever remember? My iOS devices went from instant in iPhone 1.0 to pathetic and almost not useable at times on an iPhone 6 Plus, which would still be an awesome phone for my uses (using it as an iPod essentially and the occasional call) except it's slow and buggy. Even answering the phone is no longer instant. I could have sworn I remember when Jobs introduced the iPhone he was bragging how instant everything was. What would he say now and why are apps much less stable if Swift is supposedly that much of a benefit and greater than Objective-C