Replace your Battery, you will feel it like a new phone. Yeah the difference is enormous it affects a lot the speed even the behavior of the RAM. With 80 sometimes even 85 or lower battery health the phone feels slow , always harm or hot, a lot of refreshing when jumping between apps. After battery replacement the phone feels fast , no heat or less and the refreshing is less frequent
In my experience is the battery. Time to replace it go to Apple for a replacement or go to a third party and install a higher capacity one. I work with REWA and Deji Batteries they have 20-30% more capacity and for this models with shitty battery life is awesome. P.S I work at a repair store