Performance issue in 8.6

"OpenSSH for Windows" version
8.6.0.0

Server OperatingSystem
Windows 10 LTSC

Client OperatingSystem
Windows 10 LTSC

Something puzzled me for months. Transfer rate is a third of what it should be on one way using scp. Network and machines handle gigabit ethernet normally (rates with samba are on par with gigabit eth, all drives are SSDs).

Machine A uses OpenSSH 8.1.0.0.
Machine B uses 8.6.0.0.
I tried with both OpenSSH's scp.exe and Linux's scp (WSL), getting the same results.

A sends to B: 34MB/s
B sends to A: 110MB/s
A receives from B: 110MB/s
B receives from A: 110MB/s

So basically, instead of sending a large file from A, I ssh into B and request the file from there, which is a bit annoying.

I upgraded machine A to OpenSSH 8.6 but the same happens. So I downgraded both machines to OpenSSH 8.1 and now the speed is good everywhere.

The machines, OSes, ssh keys, configurations, and the network haven't changed. I only downgraded to 8.1 to solve the issue. That being said, I do not claim the problem lies within 8.6 (I actually have no clue what's going on and I don't have the hardware at hand to try on some other systems). Maybe that's due to something completely unrelated.

Start by reproducing the directional transfer difference with scp.exe and Linux scp under WSL on Windows 10 LTSC, comparing OpenSSH 8.1.0.0 and 8.6.0.0. Measure transfers between the two machines in both directions and determine whether the regression is specific to 8.6; done means the cause is isolated or the required reproduction details are documented.