I find after about 2 years with any of my phone’s they begin to slow down . Isn’t it just a pure coincidence that the contracts are 18-24 months now , just when the phones are deteriorating .
Yeah , software upgrades can be annoying . Ya live & learn !
If you have everything backed up , pictures , phone numbers , etc I’d do a factory reset on it & restore it to the original software it came with it . Formatting a phone can remove alot of garbage that’s slowing it down .