One the performance improvements made to the 2012 product is the ability to change the level of detail on a surface based on the zoom distance. Meaning you will see a simplified surface the farther out you are zoomed, and more complex the closer you are to that surface.
This new feature can be found in the View tab of the Civil 3D Ribbon. Clicking on this command will greet you with a dialog warning you that you are entering into Level of Detail display mode. Once in the Level of Detail display mode, you can now zoom in and out on a large surface and see the level of detail change.
This is especially evident if the triangles are turned on in the surface style. I would also like to note that Level of Detail works with all surface styles (excluding watersheds and user defined contours), and also that plotting will not be affected by this command. The plotted result will plot with the “real” level of detail.
But what about editing the surface in Level of Detail display mode? This can still be done, however the edits require that you are zoomed in enough to display the “real” level of detail. If you are not zoomed in enough you will be warned with a red circle around your cursor that you will need to zoom in to complete the action.
Using this new feature you will find that dealing with very large surfaces is that much easier. Enjoy!