This may help for mouse input lag:
Edit the graphicSettings.lsx file (in the '..\Documents\Larian Studios\Divinity Original Sin' folder) in Wordpad, or other text editor, and search for 'RenderDeferred', then change the one to a zero.
<attribute id="MapKey" value="RenderDeferred" type="22" />
<attribute id="Type" value="0" type="5" />
<attribute id="Value" value="
1" type="4" />
