MalphasNuit: Bat file some here proposed didn't work.
It sounds like setting the processor affinity is to correct crashing not the lag issue, which requires either using the software rendering (easiest) or something that will translate the older graphics APIs to a newer API.
Instead of the .bat file I ended up modifying the shortcut and wanted to mention this method here since it took a bit of fiddling. This keeps the icon and lets you easily pin to the taskbar or start menu.
On Windows 10, right click on the game icon under the start menu, click on more and open file location. Right click on Divine Divinity and select properties. You will need to confirm some actions as administrator; if it gives you an error about needing permissions close properties and try again since sometimes it seems to get confused and doesn't ask for some reason.
From the properties select Advanced and select "Run as administrator". It will seem to work but not actually set the affinity if it isn't run as administrator. Then change the target field to (all one line):
cmd.exe /c cd "c:\gog games\Divine Divinity" & START "Divine Divinity" /normal /affinity 0x1 "div.exe"
Where "c:\gog games\Divine Divinity" should be the path to the game directory and can be copied from the "start in" field below that doesn't work when running as administrator for some reason. If you typo something you can change that /c to a /k and it will keep the command window open so you can read error messages.
If you pinned the shortcut it doesn't seem to update the pinned version immediately so unpin and pin again after changing properties.
After doing this if you want to fiddle with compatability settings you can do that with the properties of the div.exe file in the game directory, however I don't think any changes are needed.
To verify that it is actually setting the affinity, start the game, hit the windows key and open the task manager, click the details tab, right click on div.exe and click "set affinity". It should only have CPU 0 selected.