Try verifying local files: in the Steam library, right click on the game and select Properties, switch to the Local Files tab and then click on the 'Verify Integrity of Game Cache...' button.
Rename or delete the graphicSettings.lsx file in the '..\Documents\Larian Studios\Divinity Original Sin 2' folder, to get the game to recreated the next time it is started.
If you right click the game in the Steam library, select Properties, switch to the Betas tab and revert back to ea_version_2, does the performance and graphics quality go back to normal (after the update is completed)?
