If there was a GPU update around when this started, try resetting your graphics driver's 3D settings to default (right click an open area of the desktop to start the nvidia Control Panel or AMD Radeon Settings).
Do you have similar problems if you start the Classic version of the game? (assuming this happens early in a new game, so is feasible to test)
Are you shutting down all non-essential programs before starting the game, especially anti-virus, graphics utilities, or DirectX injectors, etc?
Try lowering the sound quality in-game. You could also try changing the Windows sound quality settings (one person with D:OS EE reported that a very high default sound quality caused performance issues in-game).
Try doing a clean boot and then test the game in fullscreen. 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.
Try creating a new Windows administrator user account, switch to that account and try starting the game from there, directly from the executable. If you can not test in a new game (if Fort Joy is fine, for example, and the issue started after getting to the next map region), you would need to copy your latest save over from your current account's My Documents folder to test.
Each save is a folder in the '..\Documents\Larian Studios\Divinity Original Sin 2 Definitive Edition\PlayerProfiles\<ProfileName>\Savegames\Story' folder.
Make sure the Virtual Memory setting in Windows is set to automatic / system managed, rather than being disabled or set to a fixed size. To check or change virtual memory settings:
Win 7/8 or
Win 10.
Make sure DEP (Data Execution Prevention) is set to default, or exclude the game:
1) open the Start menu
2) for Win 7
- right click 'Computer' and select 'Properties'
- click 'Advanced system settings'
- select the 'Advanced' tab and click the 'Settings' button in the 'Performance' section
for Win 8 or 10
- right click 'This PC' and select 'Properties'
- click 'Advanced system settings'
3) select the 'Data Execution Prevention' tab, and if applicable:
A) change the DEP setting to the first option (Turn on DEP for essential Windows programs and services only)
B) leave it set to all programs, and add an exception by clicking the Add... button and browsing to the '..\Divinity Original Sin 2\DefEd\bin\EoCApp.exe' program file
4) confirm by clicking the OK button, and reboot if any changes were made
One person with performance issues with D:OS EE on an i7 CPU fixed it by setting the game not to use the first 2 CPU cores, since Windows was running everything else on them, as well. Another was getting stuttering, and fixed it by switching the game to use only one core, applying, then switching it back to all 4 cores.
To try this, start the game, then start the Task Manager (Ctrl-Shift-Esc or right click an open area of the Task Bar), switch to the Processes tab, right click on EoCapp.exe and select 'Set Affinity...'. After that, try unselecting Core 0 and 1, apply and test the game, or unselect most of the cores, apply and select all of them again.
Also in the Task Manager Processes tab, try right clicking EoCApp.exe and under Set Priority, select High or Realtime.
