I feel its all trade offs, battery better on 15 plus, but performance defiantly is not compared to Pro.. having both
I’ve just come from a 15 Pro to a 15 Plus and the only noticeable performance difference is the 60hz vs 120hz for smoothness. Speed like opening apps etc is pretty much the same with barely anything in it. Battery life is way better as you’d expect.
Both are running iOS 17.5 beta 2.