Did anybody get anywhere with this.
I just noticed what might be the same issue.
If I just use commandline to scp a file from the server, I get great speeds.
But as soon as I start a 2nd scp at the same time, both of them tank their speeds.

There's currently nothing to test.
But, please send me an email, so I can send you back a development version of WinSCP for testing. Please include a link back to this topic in your email. Also note in this topic that you have sent the email. Thanks.

Where can I download BETA or EDGE versions of WinSCP to test if this issue is resolved?

If there is a way to run some debug command to generate useful logs, then please let me know how to do it and to where to submit such logs privately.

Buffer Optimization settings in WinSCP have no effect on this issue.

Please, let us know if the suggestion by @Guest helps.

You could try to turn off
Advanced Settings -> Connection -> Optimize connection buffer size

Windows 11 supports BBR2 congestion algorithm that can be enabled with the following commands and a reboot:

It works as it should for most applications, but enabling it cripples WinSCP 6.X throughput to an extreme degree. It affects both SSH and SFTP protocols, but only WinSCP is affected. It doesn't cripple throughput when used with PuTTY 0.78+. Any ideas?