If you're playing with HD3850 with "only" 256MB of VRam on such a high resolution, don't expect too much. The fact that your cursor feels laggy in the menu is already an indication that your game settings are optimistic.
However, if you are still at only 25fps with everything on low, that is not normal. I would normally suspect the graphics card drivers, or stuff running in the background, or a fragmented HDD, etc. Not the game.
Your setup with everything on low should run the game fluidly. If you want us to have a look at your setup, send us a dxdiag or preferably a report created with the tool below because indeed, you should get more out of this.
http://www.larian.com/hotfixes/supporttool/LPC_Divinity2.exe(Lovis Quest: talking to the priest and especially the necromancer in the cave gives you all the knowledge you need.)
