Apple iOS Background Tasks
Background tasks are partly to blame for a device's performance degradation over time - see Windows and Android devices. I have more apps than anyone on my iPhone and the performance is still great. I can just imagine if all the stupid games I download were trying to run background tasks as well. I hope Apple lets us control what apps have permission to do this.  Also I hope 'low power mode' auto-disables background tasks. Developers are going to take advantage of this in a way that often will not benefit the user.