Your GPU exceeds the recommended specs (benchmarks almost 40% faster), so that shouldn't be the problem.
Do you mean you just left the island of Fort Joy, so you are on the Lady Vengeance or Reaper's Coast?
If you load an old save, or create a new profile and start a new game, is the performance back to normal?
Try switching to windowed or fake fullscreen display mode, and/or lower the game resolution, to see if that makes a difference.
In the graphics options, if you have double buffering enabled, disable that, and if that helps try triple buffering.
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 updating to the latest Catalina 10.15.5 beta version. Apple has a signup process to be able to
get access to the betas. I'm not sure if there is an estimate for the public release.
If that doesn't help, please email supportdos2@larian.com with the sampler output from the Activity Monitor (or reply/PM/email a Dropbox or Google drive link, etc):
1. verify local files (if you have not already done so; see below)
2. launch Activity Monitor
3. start the game and get into whatever the worst state is for performance
4. Alt-Tab back to Desktop and check Activity Monitor. What do you see? How much CPU does the game consume?
5. double click on the game process and press the 'Sample' button. Wait until Sampler is finished
6. save sample output using 'Save' button
To verify 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 Files...' 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'. In Galaxy 2.0, select the game, then the settings icon at the top right (beside the Play button) and under Manage Installation select 'Verify / Repair'.
With the App Store release, or GOG without the Galaxy client:
Terminal
cd ~/Library/Application\ Support\ ... up to the Divinity Original Sin 2.app folder
codesign -vvv Divinity\ -\ Original\ Sin\ 2.app
For the first command, it may be easier to browse to the install folder in Finder, then drag and drop the location into an open Terminal window (though for the App Store version the default install path is just in /Applications ).
The second command 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
