In practical and financial terms, not much you can do exept fitting a more powerful motor etc etc.
You may want to reduce the tolerance between the duct fan and the duct tube, you can make another duct tube from
Teflon coated material, or polish up the inside of the tube to a high gloss finish, change the shape and size of the air inlet and contour of the outlet, have a more "venturi" style tube, fit flow stabilizers inside the tube (like what you see inside wind tunnels, just after the blades.), you can look at "S" shaped impellor blades, the number of blades, the pitch of the blades but by then you will end upthat it is cheaper to buy another, faster plane. All the above relates to tranferring energy more efficiently, but don't expect major improvements - except changing the pitch and the Motor.