Most likely the drivers got "updated" with Win8.1.
Update your video card driver from the GPU maker's website, then relaunch Photoshop.
Or try disabling the GPU in Photoshop and see if the framerate improves.
Running latest drivers, tried with and without discrete GPU, no change in performance.
GPU related features working well including rotate canvas as already described, just the transform functions which are very laggy.
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Also note that if you set Cache Levels to 1 in Edit - Preferences - Performance, you may find transforms to become very sluggish.
Also, what drivers are you running, specifically, with what GPU?
I ask because there are multiple sources for drivers, and some folks get them from the wrong place. The best source is the web site of the maker of the video card.
Thanks for that, that fixed the problem well!
Maybe some settings were reset on the upgrade... ?