While processing graphics are usually the biggest hit, Twilight Princess has a bunch of other things going on, especially in big fields, that you can't really do much about. If it's still sluggish, not sure there's much you can do.
Trying running the game in native resolution 480p. But there's not many ways to outright say that. Trying to play games on a Mac isn't a matter of preference, it's just misguided. Macs are not designed to run games, and you could by a PC with 5 times the power for a third of the price. But Twilight Princess is one of the most sluggish titles if your computer can't hold up. The Wii and GameCube arent exactly graphical powerhouses. But Macs have always has huge shortcomings when it comes to gaming. You may be able to run some of the less processor heavy games on modern Mac, and Dolphin has certainly be a long enough way to do most of the work.
