As said above, the translation/scene prep" time before mentalray start to render is the problem.
The translation time escalates with big scenes.
The problem at work: We have a Multisub material with 100’IDs at work and we also have large scenes with alot of mesh in it. The problem happens with SP2 or Subscription Advantage Pack installed and not with SP1. (At the moment we are on SP1 and waiting for a hotfix