Read App Reviews & Update Your Apps
If you're jumping to iOS 9 for the first time with iOS 9.2.1, you'll want to take a look at your apps. Developers have been rolling out iOS support updates for several months and these updates should do wonders for app performance.
You'll want to keep an eye on the App Store for reviews from iOS 9.2.1 users. This will alert you to potential problems and fixes.
It's still early but we should start to see reviews emerge this week and next. Keep an eye out if you value your app performance.