How is the performance if you start the Classic version of the game?
A few people have run into this issue in the DE with nvidia graphics, but Classic being ok, which can often be fixed by disabling the 'Hardware-accelerated GPU scheduling' option in the drivers.
https://steamcommunity.com/app/435150/discussions/6/1749024107678650956/Try exiting out of the Steam client, and starting the game directly from the '..\SteamApps\common\Divinity Original Sin 2\DefEd\bin\EoCApp.exe' program file, by right clicking and running as administrator.
Try right clicking the executable, select Properties, switch to the Compatibility tab and check off the 'Override high DPI scaling behavior' option (set it to Application), maybe run as administrator and disable fullscreen optimization, and set a Win 7 or 8 compatibility mode, to see if that makes a difference. Disabling fullscreen optimization helped one person with this problem with AMD graphics, in the GOG version of the game.
Try doing a clean boot and then test the game. Click Start, or hit WinKey-R, type in msconfig and hit enter; in the General tab, click Selective Startup, uncheck Load startup items (if required) and leave Load system services and Use original boot configuration options checked. Next, click on the Services tab, check the box to Hide all Microsoft services, then click the Disable All button (maybe make a note of which are currently enabled/disabled), then click OK and reboot the computer.
Run msconfig again to switch back to the normal boot configuration.
Maybe try a clean re-install of the graphics drivers. See
here or
here, for example.
