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.
Not seen this but one thing I did just notice something that may or may not be relevant. Do these machine also have an active wi-fi connection? If so does the speed get any better if you turn off wi-fi? Just looking at a different problem on a Big Sur machine where the primary network was wi-fi rather than ethernet and not sure speedtest asks or tells you which interface, does it?
I turned WiFi off for my tests to make sure it was not another variable. I only had one ethernet interface active. I ran the test with the following ethernet adapters plugged into the laptops Thunderbolt 3 port. OWC Thunderbolt 3 dock, Belkin USB-C to Gigabit adapter, Anker USB C tho ethernet adapter, Hyper Drive USB C to ethernet adapter.
All had the same incorrect speedtest results when using a web browser on Big Sur. All had different results ( what appeared more accurate results) when using the speetest app from the Apple App store.
Laptops running Catalina all gave correct results in the web browser using the same adapters.
Mac Mini's with built in ethernet did not appear to have any issues with Speedtest in a browser when running Big Sur.