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

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.