This is not exactly true. You are only "washing speed off" if you are skidding. Being on edge does not equate to skidding and in many snow conditions, carving on edge is actually faster than riding with the base flat......
Regarding that, this is also another reason to bevel the base edge during racing. Because it will "lift" the base off the snow more giving you less friction during carves.
The reason why "slalom" tolerates near 0 base bevels, is because you are so high on edge that it doesn't matter, so turn initiation is the more important performance feature. But if you're in another less turny race, it is recommended to have higher base bevel. Not just for parks.