A quick read gives the necessary information. Interestingly, cast iron is not truly pure iron. In fact, a simple steel may be closer to pure iron than than is cast iron. The difference is that steel has a carbon content of less than 2.1% by weight. More carbon makes the mix harder, more brittle, so less ductile. Cast iron contains MORE carbon than that rough upper limit of 2.1% for steel.
What happens? That carbon prevents dislocations from forming when you try to deform (bend) the cast iron. The cast iron simply won't bend. Instead, it breaks, it shatters under stress. This is why when one is removing cast iron, the best solution is often to shatter it into smaller pieces.
Ok, so what does this mean in terms of true cutting with a sharp blade, instead of abrasive cutting with a grinding wheel? If you attempt to cut steel with a tough, sharp edge that is harder than the steel, the edge actually peels a thin slice of steel away. The steel deforms as it is being cut. Thus a saw blade or a drill bit truly cuts off chips of steel. But if you look at each chip, you will see they are curved pieces of metal.
A grinding wheel is composed of an aggregate of pieces of a very hard material, typically very hard materials like Aluminum Oxide, Silicon Carbide, Diamond, and Cubic Boron Nitride. Each tiny piece of this hard material will also cut into the softer cast iron. But it cuts only a very tiny swath, resulting in much heat production and very fine particles flying off.
You should logically conclude that cutting the comparatively ductile steel is far faster, and far more efficient in terms of the work involved, since each cut produces a relatively large chip.