After four failed downloads taking many hours over my mobile wifi, I was able to download Big Sur in about ten minutes over the free wifi in a friend's coffee shop (don't ask where but it's in the UK). I was astounded by the difference. The download went without a hitch and the installation was free of problems although it took much longer as I expected. I was not paying an "emergency" data rate for the installation and simply had to be patient.
The only snag I've hit has been logging in to my banking site which required me to open a private window. That's an easy trick to learn and seems to be protective so is not a glitch.
My friend's café has wifi but the signal comes to him over fibre so it might be a good cable line is best for a successful download.