Change all the materials to just the "simple and fast" grey settings and see if it goes really fast, if it does, then its probably materials you are using. If it doesn't then I really don't know what is wrong, more then the machine choking on something.. I am not really good with MAC but if you have something like a "task manager" then check it and shut down everything you can to free up resorces. Hope that helps you out..good luck