I had the same issue at first, adjusted all the video settings to lowest, and that made it playable, but the view was cloudy and restricted. What really did the trick was the Gothic 3 community patch, supported by the developers:
http://www.worldofgothic.com/dl/?go=dlfile&fileid=166 You end up with the official Enhanced Edition, cleared up any lag and made all the difference for me. I still have to play in 640 X 480, but it looks and sounds great -- this game has a huge map and can be very confusing, so it's important to be able to see rooftops, towers, etc.in the distance to help you get your bearings.
Good luck, this is a long, long game, but absolutely epic (and read the manual, p. 22, for combat options -- I read all the time about "clickfest" and limited combat, but it's simply not true, combat is specific for every single creature in this game.)