Try deleting any shadow map files that might be still hanging around. They're supposed to be automatically deleted, but sometimes they aren't, if Poser crashes while rendering or something. This not only slows Poser down a lot, but can produce shadows of things that aren't there.

The new SR4.1 was supposed to fix this problem, but if you don't have the latest version, this could be your problem. Check your ImageIOPlugins and Prefs folders (in the main Poser Runtime) for temporary shadow map files. If you find any, delete them.