Well, not having the book and the description of what MRQ refers to as a 'heavy crossbow,' I can't be sure.
However, historically, there have been heavy-draw crossbows used that were unable to be hand-drawn to reload. Many had to be drawn using both hands and holding the end down with your feet, and with others, crossbowmen carried a small winch with them to pull the string back after firing. So it's entirely possible it could take that long, or even longer.