I have a Comp, which admittidely is a render nightmare, with 64 instances of a the same video each 1 frame ahead of the next. So it's very RAM hungry and something of a balancing act between the number of CPUs and how much RAM each is allowed (but generally allowing 3GB per process and 1 less CPU than suggested)
The thing that is killing me though is the RAM Preview is going almost 3x faster than a normal render. Even when the normal render destination is an SSD and compression is None. otherwise exact same settings i.e. 100%, best quality.
The really interesting part is that when doing a MP RAM preview the CPU usage (as viewed in activity monitor) is a solid wall of green but when I do a standard MP render the CPU useage is fluctuating wildly. I've tried various dififerent MP options (all maintaining a safe RAM overhead for the sysytem) with very little difference in performance.
This is driving me crazy.
MacPRO 2x 2.26 QuadCore, OSX10.8.3, 32GB RAM, Proavio RAID for source, internal SSD for destination.