If your cutting on the bevel of a flat polish piece of glass the olfa knife is your best and quickest. We use the 9mmSS blades and cut away. Do this all day every day as we hang tons of frosting (avery, 3M, macal, gen form, hexis). Cutting straight on the bevel is the quickest way, its all about technique and as MikePro said, slightly pull in the opposite direction of your cut keeping tension on the material.
Cutting on the face of the glass is a different story, you have to be very careful as glass can score depending on the quality of the glass. Press very lightly and use a sharp blade, if you have to snap a new blade after each cut its well worth int... you'll replace 1 pc of glass before you ever think that a blade is worth trying to make multiple cuts.
Chaz