My MacBook 12" (Macbook 12 2015 Macos BigSur). I also had the problem in Catalina. I have made a clean install, and it didn't help.
It experiences slow Wi-Fi connections constantly! I cannot use apps for streaming like Netflix or sky or any other, because resolution drops from HD 720 to 144p every time. This is very bad!
Some basic tests: Ping 2, Down 255mb, Up 23mb.
I assume there is something going on with the power saving.
Is there a way to disable all power savings? I want to have maximum Wi-Fi performance possible.