An old trick that windsurfers used to make their boards faster will probably work on kiteboards too. What you do is get some fine wet + dry emery paper and pass it over the underside of the board using water. As you go you'll feel the water starting to cling to the surface of the board, so that if you run your hand over the part you've just done it will slide, as if it's on oil. Do the whole of the bottom and the rails but be careful on sharp edges because you don't want to remove a lot of the surface finish, just rough it up.
This is also why sharks are so fast in the water because their rough skin traps water molecules so their body slides through the water.
It probably won't make much difference in a powered up situation but underpowered I reckon it will make the board a lot faster.
It's also a great way to clean up an old board.