The difference is due to rendering at 2208x1242 vs. 1334x750, which means pushing far more pixels with the same hardware (=significant performance penalty). There is no performance penalty associated with downscaling from 2208x1242 to 1920x1200 because the integrated GX6450 GPU has dedicated circuitry to perform that function.
True.
So I'm ok with the Plus having 2208x1242 native resolution with more ppi on the screen while I'd keep the 6S with the same resolution to avoid performance penalty.