http://bugs.winehq.org/show_bug.cgi?id=17956
--- Comment #3 from Casper casperbruun@gmail.com 2009-04-06 07:33:26 --- Okay, a solution is available. The slow down is caused by too many files in the Override folder. By "biffing" (removing the files from the override so they can be read natively by the game) will make the game fast again.
HOW-TO:
Before biffing, make sure you have the Fixpack installed (http://www.gibberlings3.net/bg2fixpack/) as well as whatever other mods you would like to use.
1. Download Generalized Biffing from here: http://www.shsforums.net/index.php?autocom=downloads&showfile=741
2. Extract the files somewhere (Desktop would be fine). Open the extracted folder and copy the contents to your BGII installation (probably ~/.wine/drive_c/Program Files/Black Isle/BGII - SoA). There should be a generalized_biffing folder and an exe.
3. Run the setup-generalized_biffing.exe. Follow the on screen instructions. The biffing might take a while. If the .exe did NOT ask you to install anything, close the window and proceed to the next step. The rest of you can enjoy your snappy game.
4. If you are at this step, the generalized_biffing.exe failed to update to the latest WeiDu version. Download an addon with the latest version 210 (I used UB which can be found here: http://www.pocketplane.net/mambo/index.php?option=com_content&task=blogc...).
5. Extract the Unfinished Business files to your installation just like you did in step 2.
6. Delete setup-generalized_biffing.exe from your installation folder.
7. If you don't want to install UB, just change SETUP-UB.exe's name to setup-generalized_biffing.exe and run it. It should install.
Should you want to install UB as well, make a copy of SETUP-UB.exe so you have both SETUP-UB.exe and SETUP-UB.exe (copy) in your BG folder. Change the name of SETUP-UB.exe (copy) to setup-generalized_biffing.exe and run it.