Have you tried any other, similarly demanding games recently? Performance problems getting worse like this could be a sign of hardware problems (possibly a failing or dirty fan causing the GPU to overheat, etc).
Are you shutting down all non-essential programs before starting the game, especially anti-virus, graphics utilities, etc?
Try lowering the graphics settings and resolution, or switching to Windowed or Fake Fullscreen display mode, to see if that makes a difference. Try enabling, lowering or disabling the Vsync and frame rate cap options.
Try doing a clean boot and then test the game. Start/restart your Mac and hold the Shift key down as soon as the progress bar comes up in the startup window, releasing it after the desktop appears. Alternately, from the login window, hold the Shift key when you click the Log In button, and release it when you see the Dock.
Try creating a new OSX administrator user account (either with the Documents folder in the default location, or on a different drive, if available), switch to that account and try starting the game from there to check.
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. With the GOG version, in the (optional) Galaxy client, select Divinity : Original Sin 2 in the left column, then click on the More button, and in the Manage menu select 'Verify / Repair'. With the App Store version, or GOG without using Galaxy:
Terminal bash-3.2$ cd ~/Library/Application\ Support\ ... up to the Divinity Original Sin 2.app folder bash-3.2$ codesign -vvv Divinity\ -\ Original\ Sin\ 2.app
This takes a while, and will output some lines starting with '--prepared' or '--validated'. If everything is fine, it will end with:
Divinity - Original Sin 2.app: valid on disk Divinity - Original Sin 2.app: satisfies its Designated Requirement
