Haven't done any in depth tests as those above so this is just based on what I have experienced my self. But at least for me conforming figures with a lot of polygons seems to have a huge impact on the viewport, if the "When conforming.." check boxes are turned on. Especially when you pose a character, my guess is that Poser have to constantly adjust the conforming figure to fit the character which make posing lag. However this is only the case if the conforming figure is visible, if its hidden it doesn't seem to have any impact. Unfortunately Poser doesn't have an option for hiding objects, while still making them visible during rendering. At least to me it seems that you can get away with a lot more polygons when using dynamic cloth since they don't have to conform. The down side is obviously if you have a lot of characters you will have to do a lot of cloth simulations.

What you could try if you haven't already is to use the reduce polygons (Havent tried it my self, so might not work on figures).