Cutting batts of fiberglass insulation takes forever because the batts are so thick and fuzzy. To speed up the process and produce a clean, neat cut every time, lay the insulation on top of a scrap piece of plywood with its foil or kraft-paper vapor barrier facing up. Then cut a 2X4 at least 12 inches longer than the width of the insulation. Place the 2X4 across the batt on the cut line. Compress the insulation by kneeling on one end of the 2X4 and pressing down on the other end with your palm. Then use a sharp utility knife to slice through the batt in a single stroke. Always wear gloves, long-sleeve shirt, long pants, eye protection, and a dust mask when working with fiberglass. And work carefully so the knife doesn't slip and cut your knee.