My 6plus became unusably slow and remained that way even after a battery replacement. So I upgraded to a iPhone 11 after 5.5 years with the 6Plus. I expect to keep my iPhone11 for 5-6 years at least.
I encountered this as well, the fix for me was a full reflash (from DFU) without loading the backup from iCloud. Apparently whatever was slowing down the phone was getting loaded back in. I'm guessing there's some internal cache or database or something that is grinding to a halt over time.
Same here, and my wife. That was one of my most infuriating phone experiences. The 6S was working perfectly fine then after some update it was basically useless. This was around the time of the underclocking debacle.