well, thing is, crossbows were never meant to be rapid fired.

they give more punch more easily, but traditionally just for that first shock

and a heavy crossbow is REALLY not intended for use like a barishnakov :o not even sure you CAN do a repeating heavy!

bows are a grab, pull, release

crossbows are, habitually, grab, slot, draw, trigger ; slotting taking more time than notching on a bow ... and a heavy crossbow requires the use of a winch to drwa, you can't do it by hand, and t hat's really gonna slow it down some :o