It's probably memory allocation or MP processing setup errors. There's not enough info about your project to say for sure but that's where I'd look first.
I am having the exact same problem. I purchased an Imac and it was deliverd last week. I installed CS6 Production premium and went to work in after effects on a project that rendered very slow on my 2011 macbook pro 2.2 quad core I7 with 8gb DDR3 ram and an intel HD 3000 512 MB.
My new Imac has the exact same specs as yours and I figured it would fly through the render, but in Premiere and AE the results are the same, my macbook pro is equaling and even surpassing the speed of the render time.
I am very frusterated that the preformance is like this. If someone has answers please pass them on.