If you try medium or low and drop the shadow and texture quality, does that give acceptable performance with better quality than very low?
Try setting the FPS cap in the options to 30, or something; a consistent rate may help the game seem a little smoother.
Try starting the game, Alt-Tab out, right click an open area of the task bar and select 'Start Task Manager', then in the Processes tab, right click EoCApp.exe, go into the 'Set Priority' menu and select something higher than 'Normal', and see if that helps any.
Since you have enough memory, disabling texture streaming can improve performance (it increases memory usage, which could potentially cause stability issues).
To do this, edit the graphicSettings.lsx file (in the '..\Documents\Larian Studios\Divinity Original Sin' folder) in Wordpad, or other text editor, and search for the term
Stream, then change the one to a zero 2 lines below there.
<node id="ConfigEntry">
<attribute id="MapKey" value="
TextureStreamingEnabled" type="22" />
<attribute id="Type" value="0" type="5" />
<attribute id="Value" value="
1" type="4" />
</node>
