I’m having the same problem right now.
Try going into Settings>Player>Videos and turn “Adjust display refresh rate” and “Sync playback to display” off.
That is what fixed it for me, at least for h264, which is weird because I had it turned off before by default for the longest time but when I turned it on recently, playback seemed to become smoother and I was able to play and enjoy VC1 for the first time.
Now VC1 is laggy/stuttery as hell no matter how I change around the settings which is unfortunate given Sam’s hope for better playback in Krypton.