make ssh not suck for bulk transfers

https://www.psc.edu/hpn-ssh-home/hpn-ssh-faq/

Look at this. Seems the problem isn't encryption slowing everything down but how flow control is implemented in the channels. You still need to adjust your system tcp buffers but if these numbers are right...

I know the page. But I do not know SSH and TCP enough to implement it. WinSCP uses source code of Putty for implementing the protocol. Once their authors implement this, I'll include it into WinSCP.

This issue is being tracked here.