According to the manufacturer's website, the boat is only really made to run at 20 MPH so being slow should be expected.
Now, with that said, I don't see any reason why you couldn't swap out the brushed 390 for a brushless EXCEPT for the fact that you would also have to replace the speed controller with one compatible with a brushless motor. One thing I also saw is that the plastic prop has a very low pitch on the blades, probably to keep from overheating the motor, battery pack or speed controller.