I want to make sure that you did not misinterpret my comemnts about the speed of the Little Wing 18 or the effect of its light weight.
I would guess that the Norwegian testers of the LW18 could maintain 7 knots for only a few minutes at most. They said 7 knots was the top speed and that they cruised at 4.8 knots. Other paddlers have found the speed of the LW18 to be in the same range as the QCC 700 and the Epic kayaks. You may have unrealistic expectations of a cruising speed that can be maintained for any length of time.
As far as I know, differences in weights of kayaks do not affect their long-distance cruising speeds. Light weight results in faster acceleration and more responsive handling given the same hull shape. I primarily like the light weight of my LW12.5 (22 Lbs) for the way it responds to paddler input and the ease of carrying it to an from my launch site. It is a real joy to both paddle and transport.