Hi, There has been a lot of complaints about iOS lagging, and I feel it as well. TL: DR: my 2 year old rMBP became increasingly laggy over time, especially with animations. Reinstalling OS X does little to help, but a complete format + install OS X made a night and day difference in smoothness and speed. I noticed the same thing on iOS. How do I format an iOS device? I've noticed something interesting. I've had a 2014 rMBP, and it has ran slower and laggier over the years. It has gotten so slow and laggy that many times I hated my MBP and wanted to go back to Windows. I finally decided to do something when I used a brand new rMBP, and it was completely smooth, and it ran so much faster it was literally a night and day difference when the hardware difference is minimal. So I decided to reinstall OS X since back in the old days reinstalling Windows can make your computer much faster. I originally did a reinstall in recovery mode, which just reinstalled OS X and left my apps and files intact. This did little to help with the smoothness and speed issues. I think it did improve a little, but it might be placebo. Then I decided to do a completely clean reinstall of OS X. I formatted my drive clean, and then installed a fresh copy of Yosemite on it. The result? The improvement in smoothness and speed is night and day. This is what a Macbook should be. The animations are perfectly smooth. Safari tabs open instantly. Everything is snappy. I noticed the same thing on iOS. I remember getting my iPhone 5C at first with iOS 8, and it was ridiculously snappy. It is noticeably slow now with some stutter in the animation when it should be improved in iOS 9. I've moved on to a iPhone 5S now, and I got it with iOS 9, and it was very snappy as well. But just a couple of months of use the phone noticeably slowed down. Most notably, the keyboard takes a second or two to pop up when before it popped up instantly. So, I think the cure is to completely format the phone and put iOS back on again, just like what I did for my MBP. How would I do that?