One advantage inherent in the Gazelle frame is the use of a full-diameter top tube brazed to the seat tube, versus the twin laterals in conventional mixtes, and as shown in the Herse pictures. The fully tied top tube provides better frame resistance to longitudinal twist during hard pedaling, than do the spaghetti noodle twin laterals. One may argue that pedaling efficiency or at least stiffness (remember "planing") is not a significant concern in a bike such as a mixte, but why not?

Several reasons: the large diameter tube is at least twice the diameter of one lateral tube, and hence has four times the torsional stiffness. It is tied rigidly to the seat tube, so it reacts to seat tube torques as applied by hard pedaling (really, by any pedaling). It is shorter, so even if it had the same length (full frame length, around 1.1 meter instead of around .55 m) it would be a stiffer torsion spring.

So, sometimes stiffness is good and sometimes it's not, but it hard to argue that the Gazelle will not have more torsional stiffness, and that conventionally it should be a more efficient machine.