I am having the same issue. It seems to take a little longer on even the small stuff than it ever has. I have a beast machine too, 3.2 QuadCore i7 / 16GB ram / SSD / 2GB AMD Radeon HD 6970M.
However, I do realize this is still 10.8.0 and the recent update to Aperture 3 following the realease of Mountain Lion only had back-end updates to the library and a few UI changes, but nothing noting the performance of editing and loading the masks or brushes quickly. – See release notes here
Since Apple seeded the 10.8.1 gold master to developers recently thus denoting the possiblity for an update to Ap3 as well, I am a little hesitant to do much digging at the moment. I am downloading Mountain Lion again for a prospective fresh install but am going to hold out for two weeks to see if a update addresses this issue.