I had the exact same issue. I went through every link on every result from Google by others who had this issue and nothing worked for me.
A user in one of the Apple threads suggested creating a new user profile, because that had worked for them. When I created a new profile, I got an incessant popup that asked me to login to a VPN account (I have NordVPN but it wasn't running at the time and I barely use it) -- this popup was blocking my access to my wifi though! So I rebooted in safe mode and logged into the new profile. Sure enough, the wifi was back to being fast.
I looked up info about the popup and apparently it's another known issue on Macs, and the only solution is to uninstall the VPN. So I deleted NordVPN, and the wifi was working fast on the new profile. Then went back to my main profile to start the process of migrating that account data to the new account, only to notice that the wifi was back to being fast on my old profile too now!
Try uninstalling your VPN. I have no idea what the problem was, but in the end, deleting my NordVPN app worked when none of the other solutions made any difference.