Strange, have a new iMac too, 10.8.4, 2.9 GHz I5 / 16Gb.
But i have no idea what is causing your problem. I can handle, add, multiply, split and move anything i want; the slicer will start in the background (which is nice, it is anticipating on my future actions....) and i can do whatever i want while it is doing so.
My iMac is running 4 desktops, one with email and chrome, one with Cura, one with Eclipse including 2 emaulators (which is quite heavy) and one with 2 flash-games (yeah, i know...).
So, i can not confirm your experience.