Why is it that racing drivers actually speed up when traversing a curve?

Because the classic racing technique is to do all your braking in a straight line before turning in to the corner, and then drive the car on acceleration into and through the turn. This basically uses the centripetal force of the drive wheels to drive to the apex and around the corner and counter the the effect of centrifugal force. The net effect is that race cars pick up speed as they go through corners.