I had sluggish response with smooth shaded previews with my GF8800GT.
Settings views to flat shaded helped a lot with large detailed scenes.

You can try dropping the setting for opengl from settings down from shader 4 too.

[edit]
just tested some more. 
I get rid of the sluggishness by dropping the opengl settings in options.
From opengl 2.1 (shader 4) -> opengl (fixed hardware pipeline)
[/edit]