Hi. If you're getting warnings about running out of RAM, then that's definately one of reasons to slow rendering - even very slow rendering.
Another reason often seen, is "wrong" or "bad" material set up. It's not always enough to convert the materials automatically from VRay, so check out these materials.
Unlike most CAD-programs 3DsMax doesn't handle hundreds of thousand objects very well, just like it doesn't work well with objects far from drawing origin. So keep as much as possible as instanced objects, merge objects together (not the same as grouping), and make sure that the whole model is close to 0,0,0
Hope it helps