I noticed my new IPad Mini 4 was getting extremely slow downloading data files. I am running IOS 9.2.1. After reading numerous threads on the internet I found the solution is to press: Settings - General - Reset - Reset All Settings. After doing this, my IPad Mini 4 is lightening fast doing downloads. Of course as you change your settings back to where they were (as needed) your IPad might once again become slow. I'm not sure which of the settings is causing the drastic slow-down.