but after enabling 4x multisample antialiasing the frame rate dropped to 10 fps.
^-> this, my friend, has nothing to do with "multitasking". The game's engine just not optimized for any form of "true" AA (FSAA/MSAA,SSAA, etc), thus, the performance is terrible. Also, it's just your imagination: there is no difference between multisampling AA or Supersampling AA in the game. Just because AA does not work at all.
