i was actually having issues for awhile trying to force 45fps with entering it manually into the MixedRealityVRDriver settings, it eventually kicked in not sure what was the go, problem was i had to reset it when i was playing other vr games to get back to 90fps etc, i just stumbled across that other way of doing it while in game with the controller, and it worked and have not needed to change it, but doing it from the actual steam settings itself before loading in a game does not work for me, only changin ss will work so not sure whats going on tbh