Boi
I found it was Microsoft Teams client that was affecting my PC after Win 11 upgrade.
I happened to notice after many network resets and reboots that only Teams was running, so I killed all related processes and the internet speed returned.
When I ran Teams client again the speed reduced to 7-8 MBPS. In the end I removed the app after several tests to check it was this. Note: never happened in Win10.
Some clients like Teams - so have to use the Browser version. It is a shame as cannot auto set up in Outlook.
If you find the magic switch in Win 11 that is doing this let me know.
Martin