Agree with Walther on how the polys are organized on the models. I have had some fairly low-poly scenes with imported objects that slowed to a crawl, even with grouping, layering, and hiding layers. And some large-poly scenes that still tripped along just fine. I find it's not only polys that can cause my scenes to start slowing down, but number of materials as well. A lot of my scenes have a huge number of materials in them, and even with a lower and efficient poly count, they can start to stumble a bit. So I save in increments just in case.

Sgiathalaich