Support the material against something solid and cut with a slicing draw rather than a squeeze. For zip ties, flush cutters or the wire cutters on a multi-tool are cleaner and safer than any knife. For rope, a serrated edge or a hooked blade beats a plain edge because fibers roll away from a smooth edge.
A knife works but leaves a sharp stub that catches skin. Flush cutters or multi-tool wire cutters cut level with the head and leave nothing to snag, which is why electricians use them.
If you must use a knife, cut against a solid backing rather than freehand, and cut away from your hand.
Fibrous material rolls away from a smooth edge, which is why plain edges struggle once they are anything less than very sharp. Serrations trap fibers and shear them, and a hawkbill pulls the rope into the cut.
Tension helps enormously. Rope under tension cuts with a fraction of the effort of slack rope.
Draw the blade rather than pressing. A slicing motion uses the length of the edge instead of one point of it, which cuts faster and needs less force.
Never cut toward a hand holding the material. See can a dull knife be more dangerous.