Well variables like wind conditions and what part of the wind window you are in also play a huge part when you are learning. A little swirl will cause you issues and you may not realize the swirl was even there. those blessed with smooth beach winds will know exactly where I'm coming from here.
The design of the sail, type of bridle and finesse of your technique also makes a lot of difference. Some combinations just back up better than others. And as your technique gets better, so will your speed. Be patient it will happen.