Ulco's suggestion is probably one of the best ways.
At working time the scene is very sluggish with some small changes even in the nodes.
If you use the wireframe preview, it is the best to pause it so much you can and only start it when absolutely needed.
If you have finished in your mind how your scene will be rendered, you can try to delete polygons that are facing away from the camera for example. There is mostly no need for polygons of an spaceship on the far other side for example.
that Said , still Terragen handles very big textures and objects very nicely.
I wish i had time to make those kind of changes for my objects in the VR contest before.
I think Oshyan is remembering me much if he had already time to look at the scenes