Preface: I'm using a 6, and went through the entire beta cycle, fresh installed the GM, then updated to the final post GM update the day of. Like a ton of ya'll, I experienced lag going in and out of apps, and all over the place. Sometimes it'd go away, sometimes it was glaringly obvious. I got frustrated with the spontaneity of my 1 year old phone performing oddly sub-par, and today seemingly fixed it and hopefully can help anyone else. Last tidbit - I use iCloud backups. 1. Made iCloud backup 2. Connected phone to iTunes via USB 3. Hit restore. Not update, not shift+click anything, no IPSW downloads, just restore. 4. When the phone came back on, restored from iCloud backup 5. An hour later, everything is smooth as butter. I know everyone lends anecdotal advice for fixing this type of ****, but the difference in my phone right now after finishing the process above, and prior to doing it is night and day. Hopefully this helps at least one person get by with iOS 9! ** Just to add on, I was the biggest critique of the half second delay when going out of an app and already tapping a new one to open via the homescreen, and i'm not longer experiencing it.