In my experience drumstep tends to just be a cross between dubstep and DnB, whereas half-time is usually just DnB but at half the tempo. Some of it almost could be considered hip hop instrumentals but with the sound design of DnB. The two groups in that scene that come to mind are Ivy Lab and SHADES, though lots of DnB and IDM artists have half-time tracks (voljum, Noisia, Chee, Kursa, Seppa, and so on).