If you can get any of the work, chess960 units only took about 12seconds on my PentiumD....
After that, erm, well, em. Spinhenge? At just under an hour a unit it's the next shortest I can think of.
There used to be a graphic in one of the threads that showed info like this, but I think it fell foul of the 'pruning' that was done when the forum got unmanageable.