Have you checked with GOG on when they will make the patch available? They make their own installers, so once Larian gives GOG the update, the rest is up to GOG.
Here is an
unofficial alternate download (1.8MB), containing the 4 files in the patch (div.exe, DivDialogSystem.dll, Osiris.dll, slash1.dll). Extract it into the Beyond Divinity folder, overwriting the files there (make a backup first, if you are so inclined). The archive is in the
7-Zip .7z format (rename to .exe if you don't have an archive manager that supports this format). See
this GOG topic for details.
If you get away from the steam vents on the initial level of the game, does the performance improve any?
