AnyaB: Since I found this forum, searching for my own Windows 8.1 Arcanum lag issues, I thought I might mention how I fixed mine (so far, as it appears).
I have GOG Arcanum installed as well as the Unofficial Patch, High Res, HQ Town Maps.
1. I overwrote my existing Arcanum ddraw.dll with the DDraw Fix.
The ddraw fix can be downloaded alone by searching for Aqrit's DDrawFix.
I didn't use the provided exe, simply extract, cut and paste into Arcanum folder, the following;
ddraw.dll
wined3d.dll
libwine.dll
2. Then add -doublebuffer -no3d to the Arcanum shortcut after Target: "Arcanum.exe"
(Mine looks like this "C:\GOG Games\Arcanum\Arcanum.exe" -doublebuffer -no3d)
I don't know if all the .dlls are needed, but since everything seems to be working fine now. I haven't bothered to further experiment.
Also NOTE: This doesn't require Arcanum to be run in a window.
The -doublebuffer -no3d alone didn't work for me, neither did renaming or removing the existing Arcanum ddraw.dll. And, the ddraw fix alone didn't work without said shortcut addons.
I came upon this all by accident/experiment after looking at Baldur's Gate which runs fine on my Windows 8.1 and found the ddraw fix folder...
Hope this helps someone else.