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.
To cut out a notch, like around a door or window, you'll need a drywall saw. Cut the shortest leg(s) of the notch with the saw and then score and snap the longer one. If you have to cut out a hole for an electrical outlet or switch, use the drywall saw.