Why does anhydrite drill slowly?

I am not quite familiar with that phenomenon (I am used to boring or coring in soils rather than hard rock applications), but I would put it down to plasticity acting to slow penetration.  Muddy soils can do the same general thing when compared to sands.  The gooey-ness of the material acts against the free movement of the borehead or drillhead.  It does not cut well, it tends to smear and gum up the teeth or auger and thus resists passage of the drillhead.

To penetrate requires being able to displace the materials, and plastic-like materials don't turn into easily displaced particles.  They don't get out of the way easily.