Interesting question.
I wonder if we are getting the distinction between maximum gliding distance and minimum descent rate confused?
Drawing from my glider experience, there are two important speeds-- the so-called "Best L/D" (Vmp) and "minimum sink speed". Best L/D, as mentioned, is proportional to weight-- a higher weight will require a higher speed to be able to fly the maximum distance. It is the best compromise between forward penetration and altitude loss.
However, when flying in areas of updrafts the speed would be reduced-- to minimum sink speed. Minimum sink speed would provide the lowest descent rate (in feet per minute, knots, or whatever unit you prefer tu use).
If two gliders, at the same weight were flown in still air, the one maintaining Best L/D would have a higher descent rate than the one maintaining Minimum Sink speed. Because of this, the glider flying at Best L/D would land before the one flying at Min Sink. However, the glider flying at Best L/D would cover a much greater distance than the one flying at minimum sink speed.