I have pinned it down to both my SB2 and Ubiquiti Amplifi Router (Mesh Router!!!).
Some observations in my case:
(1): It seems that in task manager the speed is stated is definitively at odds with the adapter status information (Network usage always 100% and inconsistent with status info of network adapter [e.g., I am downloading at 100 Mbits and adapter says 24 Mbits)].
(2): Also, when I am only some meters away from the mesh point next to me, the speed goes down faster than in other Notebooks (My 4 year old Gigabyte is faster still, in terms of WIFI speed).
(3): Non mesh routers (like TP-Link 5G types) are not causing so much fluctuations in the speed, as I could test with my AC1750 Wireless Dual Band Gigabit Router. I can't really blame the Ubiquiti as SB2 seems to be the only notebook affected!
(4): The more devices are using the bandwidth (even too a lesser degree than SB2 itself) SB2 will reduce the speed significantly!
All-in-all: no solution presented here so far was relieving the major flaw of SB2 (again Wifi as in Surface Pros). I am heavily disappointed by MS! I may return this piece of low quality 'thought put into' electronics.