The best idea is to set the number of threads to four (even if you only have a dual core). Only reduce it if you encouter memory problems. The reason is the way Poser distributes the image to the processes: it assigns them at the beginning. So if you start with two processes the image will be divided in half and then rendered. Now if the first part takes two hours on one core and the lower part just 30 seconds, you'll have to wait the full two hours. If it's divided in four parts, you may only have to wait one hour. (Of course it could be two hours still, because now one of those left two parts (of the four) still needs two full hours and the other part nearly none.)