I got an app that is used throughout the day for collecting signatures. The current release is used by my customers without problems for months on a daily basis. But recently, some customers who updated their iPads (iPad 4 Retina in that case) to iOS 9.1 reported that the app is unresponsive when they start to use it after a night of charging the iPad on a Mac. It takes several seconds to respond to a touch in a list with just a few menu items and signing is not possible at all since the touch events are recognized only every few seconds. The app does not crash until the user gets annoyed enough and forces it to close.
The app is usually the only one used on the iPads. It is used heavily during the day and the iPads are not used at all during night (just charged).
I cannot duplicate the issue on my iPads or in the simulator. I don't see any leaks on Instruments or other problems in the debugger.
The app is build for iOS 5.1.1 and newer (so that it can be used on the original iPad). The App Store version is build with Xcode 6, a beta version, which shows the same behavior (at the customer), with Xcode 7.1. I use Core Data for local storage and CocoaAsyncSocket for communication with the server.
Sorry for being so vague, but does anybody have an idea where to look at?
This problem vanished after Apple updated iOS to 9.2.x, but I don't remember the exact version.