I just received my M1 Mac Mini. I was experiencing the same issue. I have 100mbps connection and when did the speediest I was not getting more than 20mbps. At the same time my old mid 2011 Mac mini and latest iPhone, iPads were showing above 110mbps.
After trying a lot of things suggested on internet, I finally came to this thread.
After reading some of the responses, I tried to adjust the direction of my new mini and found that when Back side of the mini (where ports etc. are) is not facing wall and has empty space in front of it or above it, I started getting 116mbps.
I tried with exposing the bottom of mini as well (since someone suggested it), but it didn't help.
I can reproduce the problem with keeping port side of the mini facing a wall ( with less than a foot distance between two) and then problem goes away when port side of the back is not facing the wall but backside front or up.
Why should it matter? Its really inconvinient to place the Mini the way I have to now to get Wifi to work. Now I have to buy a mount or something...