It's not like I've ever timed it.. but I used to have a big monitor in my modest studio, the models loved instant feedback, and they came across fast enough to help them adjust their poses. When I'd look (after adjusting light or some other change) I remember making the capture, immediately looking at the screen, and then waiting 1-2 secs for it to appear. And I'm talking raw with a 1ds2 or 5d2.. I used a 1ds3 briefly and don't remember there being a difference. Remember, a preview must also be generated in any viewer.. so processing power comes into play for that, and video card speed for rendering..
If you think about it, it takes almost as long on your cameras LCD.. press the display button and 1-2 seconds later its there. An extra sec to the computer and screen sounds about right.
Personally I don't think you need to worry about it, your computer will buffer as fast as your camera can take the pics.