Had the same problem with my iPhone 6 Plus, the performance was barely tolerable on iOS 11. I think it might be because Apple skimps on RAM so that a few generations down the road it's just not enough anymore. Upgraded from the 6 Plus to a 7 Plus that i got for cheap from a friend and it is A LOT snappier. The 7 Plus has 3GB of RAM, the CPU is also twice the speed tough, which makes a difference as well.