If you are not getting optimal performance while playing on a MacBook here is something you can try:
1.) Close the game completely.
2.) Open up the Baldur.ini (if you are using v1.3 or v1.4) or Baldur.lua (if you are using any version above v2) file located in: ~Documents\[Game Name Folder, e.g. "Baldur's Gate - Enhanced Edition"] in your favorite text editor.
3.) Paste the following lines after "INSERT INTO":
'Graphics', 'Postprocessing', '0',
'Graphics', 'Shaders', '0',
4.) Save the Baldur.ini file and run the game. The lag should be gone.