Its a well known issue of this type of design: everything else remaining the same, a bigger ratio between the input rotational speed and output rotational speed (it can be a reduction or multiplication) reduces the efficiency.

There are no measurements for the BWR vs. the BSR because the BWR is a proprietary IGH made for Brompton but you have exactly he same effect for the Schlumpf: the Schlumpf Mountain Drive and High Speed Drive with a 2.5 ratio have a significantly lower efficiency than the Speed Drive with a 1.65 ratio.

More speed doesn't necessarily mean less efficiency, it depends of he IGH design, the 14s (actually 2x7s) Rohloff has a better efficiency than all other IGH with less speed.