you should not have upgraded to 8.1, it sucks and doesn't run any game, i know alot of people who upgraded to 8.1 and literally ruined their pcs by doing that.
i guess u haven't really used windows 8.1 or windows 8 for that matter. windows 8.1 is very fast and avery good os for gaming. I m using 8.1 and play dota 2, battlefield 4, nfs rivals, dirt 3, without any issues.
maybe when u have installed it u didnt do it properly or something.