Do you have a spare wired mouse you can test?
Try disabling the deferred renderer for the game; that has helped some people with input lag, though it is more likely to cause a crash on startup than help. It is easy to check, though, and easy to changed back.
In the '..\Documents\Larian Studios\Original Sin\graphicSettings.lsx' file, search for the term 'RenderDeferred', and in the second row after that, change the one to a zero, and save the file.
<attribute id="MapKey" value="RenderDeferred" type="22" />
<attribute id="Type" value="0" type="5" />
<attribute id="Value" value="1" type="4" />
Check that DEP (Data Execution Prevention) is set to default, or exclude the game:
1) open the Start menu
2) right click 'Computer' and select 'Properties'
3) click 'Advanced system settings'
4) select the 'Advanced' tab and click the 'Settings' button in the 'Performance' section
5) select the 'Data Execution Prevention' tab. if applicable:
6A) change the DEP setting to the first option (Turn on DEP for essential Windows programs and services only)
6B) leave it set to all programs, and add an exception by clicking the Add... button and browsing to the '..\Divinity - Original Sin\Shipping\EoCApp.exe' program file
7) confirm by clicking the OK button