I took my 6 Plus to my local Apple Store because I was having so many problems beyond just lag. Most problems started after 9.0 but some were there before. They swapped out my phone that was still under warranty with a phone on 8.4.1. For some of you having major issues it might be worth a shot to get it swapped out with a new device. I don't know if it was just luck that it was on older software or if all replacements are on 8.4.1. This phone is very smooth with no lag or glitches, clearly there is something wrong with iOS 9 for some devices.