I wont say the value is somewhat random when the quad is above VPS range (from 0 to 3 m) but it seems to be.
I'm flying only outside for the moment, I set VPS to OFF. Usually, when the quad is above 70m height, having an horizontal speed or not, I always have 25,5 for Hs. And sometimes I have this value above 40 or 50m.
Horizontal speed is only available in the flight log, not in the subtitle file.