Its the hardness of the material that will slow down the drills, not the density. Metals such as gold are quite dense (high specific gravity), but they would be easy to drill because they are soft.

Remember that the cutting surface of diamond drill bits are just a slotted rim of industrial grade diamond pieces bonded together. The surface is like the coarseness of sandpaper. Like sanding wood, when its soft its easy, and when its hardwood, it takes longer, generates more heat, and wears out the sandpaper more quickly. 

I doubt the breakdowns they are experiencing are due to drilling conditions. Its more likely the condition of the drills that's the issue, which explains why Nemis would be buying 4 news drills.

Regards,

B.