Why do you have a low boom speed sailing?
Sailing fast is all about taking your weight off the board and taken up by the sail. This reduces the drag on the board.
Historically tall sailors have been faster, not always the case though Eric Beale & Olivier Auge weren't the tallest but very fast, however they are out numbered by the tall fast sailors, Pascal Maka, Bjorn, Anders, Roddy Lewis.
Being tall allows your center of gravity to be out over the water and off the board. Lowering your boom does the same thing, it lets you hang out further over the water. Jurgen is the the best I've seen at this.
It isn't always the best option however. When the sea is rough you won't be able to keep control of the rig and the board will bounce all over the place.
Canal conditions are the only occasion when you can drop the boom real low. Also when the wind starts really blowing then you'll want to have more control with a more orthodox boom height.
Any more queries add a comment and I'll see if I can answer.