The preview pane updating is what generally causes those sorts of things, especially with heavy textures or geometry loaded. Simplifying that display by lowering preview pane texture quality, lighting quality, choosing a different renderer for the preview pan or using things like bounding box rendering, instead of true geometry, can significantly speed up your Preview pane. Poser has a very good number of options, settings and display styles to help with working with crowded scenes in the Preview Pane.

IMO, the real killer regarding scene overloading is textures. One can see the problem pretty easily when one loads up a scene with quite a few figures in it and stuff starts to turn black because one's vid card just doesn't have enough texture space. Texture optimization is key if you're trying to save video processing space. By that, I mean that if you have several of the same figures in a scene and only really need to change their face texture, taking time to match that face texture with a commonly-shared body texture will dramatically improve performance, since Poser doesn't have to set aside space for yet another giant texture. One can also save some room by using seamless repeated textures in materials, where applicable, instead of trying to load up one big texture for something like a plain carpet, for instance. Anywherere one can use the very nice abilities of Poser's material room to replace the use of large raw texture files should be taken. Material generated textures are fast. It's the extra effects that get tacked on that may make them take a bit of time during rendering. But, in the preview room, it's a no-brainer -Use 'em!