I believe it's the kind of advanced tech that can be designed after a couple doses of spice. Before the folding space thing, but after the mentat mantra.
Also, in the Lynch move during the fall of house Atradies, both sides were mostly using projectile (or at particle beam) weapons, which leads me to believe the personal shields were expensive, and tended to be used on higher ranks and royalty, and the slow rounds to defeat them were likely also scarce.
The anti shield round seems to travel quickly (at least at normal "projectile speeds) until it encounters the shield, then it slows down and drills/powers through. I always assumed that it was self powered (like the hunter/killer) and when it got to the person inside the shield it just continued it's slow advance under power, not needing mass or velocity to penetrate.