@kewe65: Thanks for sharing your findings. What version of Windows is that?
Before posting, please read how to report bug or request support effectively.
Bug reports without an attached log file are usually useless.
netsh interface tcp show global
scp from the same file source gives at least 10x faster speed on the same machine, same everything (encryption, CPU intensity, blah blah blah -- it's all the same). And no, debugging is not enabled in WinSCP. I noticed that WinSCP starts at showing 2.9MB/s then in a few seconds it settles down at around 600KB/s, which is just ridiculous for speed.
scp from a cygwin command line prompt and it was going at almost 8MB/s, which is what I was expecting, and the file was here in 3:02 – that's three minutes). That's unacceptable for WinSCP to have such a poor performance, no matter how fancy the UI looks like.