Most of that slow down was not caused by this power management issue. You NEED to pickup the updates the fix the issues that caused those problems. New batteries will not be affected by this. . . and there are IMPORTANT SECURITY updates in the new iOS update your iPhone 7 needs.
I noticed a slow down on my iPad Air after updating these last (numerous) times. I turned off this and tweaked that, but you can tell response times are slower just moving around the pad. I have no battery issues. The weird thing is my older iPad 2 seems faster (they quit updating that awhile back).
In general, I’m unimpressed with where we are today re: speed. Like my Dad used to say, “When I click on something and BLINK! it’s there, my computer will be fast enough.”
I’m going to coin a term: Latent Dissolve Fatigue, for the gazillion times one screen dissolves into another, fades in and out, etc. Every minute little fancy delay adds up, over time, to LDF.