My approach is to set the base scene (in your case, the warehouse) and set up exteriors (stuff visible through windows), skydome, sunlight. Do test renders until I am satisfied with the lighting. At that point, when I start populating the scene, I change to wireframe mode. I have scenes with over 500 objects and characters (many are between .9 and 1.2Gb files) and can move around in it just fine. I could do so on my i5 laptop as well. The problem is that Poser needs to load the textures for everything you bring in to the scene, and if you move around in the scene, you are asking Poser to recalculate light and shadow for everything. That would be my guess as to why the preview screen is slow and jerky. Take away the lights and textures by using wireframe, and that problem is gone.

I also save my scenes in wireframe, so when I open them, Poser doesn't have to load textures. The scene loads an awful lot faster.

Before upgrading anything, give that a try and see if it speeds things up. To be honest, I hated wireframe at first, but I soon got used to it and find that I now have a better idea of what's going on in a scene than I dod before with full shadows and textures.