To reduce speed loss, you can limit one or another kind of drag.
Good tests are to get the right speed first right before the wind, and then with the wind on the beam. Before the wind, you are only dealing with being pushed by aerodynamic drag and slowed down by hydrodynamic skin drag, so it's only two things to balance with each other. With the wind on the beam, you are only being pushed by aerodynamic lift, but now you will be seeing some form drag. (The "form drag" should come in to play when you have some angle of attack with the water.)
The water lift coefficient controls how big a side force you get from an angle of attack with the water, and the "griping" slider controls how far forward is the point where this lift acts on the hull.