Is there anything I can do to make Roller Coaster Tycoon 3 operate faster on my imac?

My imac is the standard white one, I don't know what it's called. I have leopard installed and this game is unplayable. I had the first and second for PC and the speed was MUCH better. It wasn't even a high-quality pc. I've already turned all the graphics settings down and it's still horrible. I just bought it yesterday and i'm already thinking abut getting my money back. The apple store also charges $50 for it when its real value is only like $20.