With straight leading edge weed fins, the rake has to be set for the span-wise flow to pull seaweed along the leading edge to the tip. This causes the lifting surface in the tip to be located a long way behind the centre of rotation of the board (trim axis). This is not really a problem deep off the wind with short speed fins, but as you go larger the problem becomes worse. In the end, the straight leading edge is a big compromise in balance. Upwind or across the wind, where the centre of pressure of the fin is too far back for optimal balance with the rig, is where you notice this the most.