Hi I had a similar problem and it was caused by the students/staffs redirected start menu from our Windows 7 GPO, set up a windows 8 OU and put the windows 8.1 clients in there, created a loop back that redirected everyone's start menu to the local profile, no roaming or mandatory profiles takes about 40-45 seconds first logon on a windows 8.1 Tablet over the wireless, then it is really fast the next time it is used. Works for me for now as I only have a 30 windows 8.1 tablets not sure if it would be useful with a full roll out, I have tried to export a start screen layout but that is not taking, so the desktop needs shortcuts to needed programs, but the desktop redirection to a network share still works as in windows 7 so OK.