We usually will worry for performance drop in major OS updates, specially for older devices. Since the update of iOS 8 (and the subsequent iOS 8.1), iPad 2 and iPhone 4s users have been complaining about their devices being too slow. So Apple has pushed out the iOS 8.1.1 to address this issue and to squash some bugs in the OS.
iPhone 4s and iPad 2 are already few years old and they are running on Apple A5 chip so it is understandable if they are running slower. However, the first iPad Mini is equipped with the same chip and it has better performance so I guess with some optimization effort, the older devices can have a boost too, if Apple has no intention to force you to upgrade to a newer device.
I do not have both of the listed devices to try so I cannot really tell how much difference will the update bring. For my iPad 3, the update run smoothly and I can hardly tell if there is any improvement. Luckily, I have not encountered any problem yet.
[Update] According to the report from iMore, this update is giving back users up to hundreds of MB of storage for iPads. Some readers reported the same for iPhone. We are not sure what Apple did behind the scene. Nevertheless, it is a great thing to have more storage.