I work my videos in a Mac Mini, 8GB 2.5GHz Core i5. I use After Effects CC. This mac can manage 1920 x 1080 video, you only set preview to Half and Skip some frames then I feel good with this mac.
But it is too slow for rendering 3D objects. No matter how you set your previews, it definitively cannot do the work. It can take over than one hour per second of Full HD 3D video.
I'm thinking about an iMac which brings the GPU unit. This mac is more than twice the cost of my actual. My question is how faster this iMac will be in comparison with Mac mini, in order to estimate if this expensive investment should be redituable.