Duke William of Normandy won the battle because was well prepared and had a good army. They prepared carefully for the battle. The Normans had knights on horseback who were skilful fighters.
A utility knife, metal ruler, and a hammer can be faster than trying to carefully saw an opening. Score the drywall a couple times till you are most of the way through, then just knock a hole with the hammer and break the drywall back at the score lines.
Measure and mark the cut line at the vertical midpoint of the drywall sheet. Use a straightedge to help guide your utility knife as you cut. Tip the panel off of the floor or table at a slight angle, then push it down to break it in half. Use the utility knife to cut through the paper backing.