I experience like a lot of you a GPU (AMD 6440) working despite I need to use an old AMD drivers (on Windows 7 system), but the result is obviously without any benefit!
Example, for a zoom it can add 2-3s with GPU acceleration ON!
And this night, I have got an idea... and I tried it this morning: Disable the anti-virus.
And it's working in my case. Now with GPU acceleration "ON" I obtain a speed a little bit faster on zoom, but not really with brushes (not a big difference especially if your are going slowly).
My anti-virus is Kaspersky 2015.
Now I have reactivated it but add "Lightroom.exe" in the "Trusted Application List" (not sure of the English translation).
If you have others tips...
> I have got an idea
I have got one too = fix the ACR/LR code ...