FWIW.. Any abrasive disk I've used in a chop-saw has accomplished the cut much more quickly when the disk contact is at the thinnest cross section of the material.. apparently not the situation with the rock drill rod, just commenting wrt general shop practice.
Thick square, round, hex, etc cross sections are just generally slow (cut more effectively with a saw and a coarse blade), but any flat stock cuts faster when the disk is approaching the thin width and not the wider surface.
The larger the contact area of the disk's edge, the more heat is generated in the disk, which I believe also reduces the shedding of the dull abrasive grains.. so the cutting action is significantly reduced, or so it seemed to me.