Hello,
I discovered an unusual issue this week and was wondering if anyone else has seen this problem?
I have tested this on two 16" 2019 MacBook Pro's running Big Sur. If you run a speed test from https://www.speedtest.net or any other speedtest site in any web browser on Big Sur, the upload results are way, way off, typically the results show half to a third of the actual speed. The same test on MacBook Pro same model running Catalina and it returns accurate results. I tried SEVERAL different ethernet adapters and experienced the same results. I did notice Intel and M1 Mac Mini's with built in ethernet running Big Sur did not have this issue.
The Speedtest app from Ookla from the Mac App Store appears to return accurate results on MacBook Pro's running Big Sur with external ethernet adapters.
Earlier this week, our office upgraded our internet circuit and I "spent hours" trying to figure out why our speeds were so bad using speedtest.net which has always been relatively accurate. I assumed the problem was with the new ISP circuit or a QoS policy... I finally figured out it was any MacBook Pro using an external ethernet adapter and Big Sur. Ouch!!
Hope this helps someone before they fall into this trap.