No, you had 20+ applications open. Not that many were running.
Since most gui applications are event driven, the vast majority of your applications were simply waiting for you to interact with them.
Only the game will have been running constantly, the music player will probably have been executing code mostly upon interrupts and spending the rest of the time asleep too.
Try running one cpu intensive program that is always in the "ready to run" state, at the same priority as your game and see how well you can play it...