The "speed", or more accurately group delay and decay, associated with sealed subwoofers is not a universal truth. For example, viewing the graphs for the
SVS 20-39PC+ (12Hz) and
Velodyne DD-12 show that the group delay is only a few ms better for the DD-12 in the higher subwoofer frequencies, and actually worse <20Hz. Decay is slightly better for the DD-12 across the entire spectrum. The
B.K. Monolith DF has very good group delay <20Hz, comparably, but distortion increases too rapidly. Decay seems comparable to the 20-39PC+.