Dicko is on the money there, after just a few years of paddling I have quickly come to the conclusion that there is no "fast" ski
It is all relative, perhaps the best question is what ski has the best hull speed disregarding stability issues, because unless that hull speed translates to results on the water it's just not fast if you get my drift.
When a top paddler from Western Australia went to a coach in the Eastern states for coaching the first thing he did was sit him in a ski and throw a ball to him and ask him to throw it back. You would think he would be looking at his stroke technique... the point being that the more stable you are the better hull you can paddle and the more power you can apply to the stroke.
I continue to see real mixes of ski's at the pointy end of races outside of sponsored paddlers (say 5-20th positions) because some paddlers can make a hull work while others are fast in what some consider a slower hull.
This is part of what I love about the sport, finding the right mix of ski for your ability, which may improve over time, hence why some of us can change skis very regularly looking for that incremental improvement.