Cast iron cuts better with a bit that has had the hook taken off the
edge. A drill bit can be modified by grinding the cutting edge's flute
to a surface that's aligned with the bit's axis rather than having the
slope created by the helix. Such an edge won't snag and break the bit,
and cast iron (gray cast, anyway) cuts best when it's pushed a little
rather than scooped out as a normal drill will do. And a modified bit
has a thicker section through the cutting edge and doesn't heat so
quickly.
HSS lathe bits will cut cast well if there's a neutral rake on
them.

Dan