I had completed BG:EE on the App Store, then realized this. If you plan to play the Siege of Dragonspear (SoD) DLC-which I strongly recommend by the way-I suggest you actually buy the whole set through Steam, as I haven't seen a way to get SoD through the App Store. Check Activity Monitor for culprits, as Big Sur compatibility, as of right now in Dec 2020, can still be rough for some apps. I've only experienced a couple slowdowns but it was due to other apps eating up all my RAM (looking at your Google Backup and Sync). It even takes up my entire ultra-wide monitor in-game for a more cinematic feel!
It now runs beautifully and incredibly smoothly. Before I did that, I had issues getting past the main menu. In the window that appears, under "General", be sure to set "Open in Low Resolution" to TRUE. The KEY piece is I needed to go to Finder → Applications, right-click on "Baldur's Gate - Enhanced Edition", select "Get Info". I had some reluctance given some other reviews about crashing but really wanted this to work, in anticipation of the full release of BGIII.