The number of figures/props/textures you have installed really doesn't make any difference. What matters is what you have loaded in your scene. Hi-rez figures such as Mike and Vicky, high rez props, high rez textures or having multiple figures loaded can take it's toll. Poser can also be slowed down if you have a lot of other programs running and/or a resource hogging app (any Microsoft product). If you haven't defragged your HD recently, that can also slow Poser down.

Hope this helps.