I have just upgraded to Windows 10 and get exactly the same problem now.
Update: After searching the internet for more info,
I tried to uncheck the GPU box but this also caused Lightroom 6 to freeze, thus needing a restart.
I checked the graphics driver, it was already up to date.
I disabled the Graphics card via device manager and all seemed to work, also I was now able to uncheck the GPU box.
I then closed and relaunched LR6 and checked the GPU box remained unchecked.
I then re-enabled the Graphics card, closed and relaunched LR6, GPU box remains unchecked and, fingers crossed, no more freezing.
This is not a fix, just a workaround until a proper fix is found.
I hope this is helpful to anyone else who has this problem and also to the guys at Adobe who try to fix these niggling problems.