I have tried this on three different servers now and on all three show the same behavior. I tried changing the number of simultaneous processed files, no change. I have tried SFTP and FTP with TLS and without, no luck.
I also found out, that speed loss is diretly linked to the amount of files processed. One file will show no loss at all, two will show a little loss, three some more and so on...
I love WinSCP but being forced to pack everything into a single zip, to be able to work with it, is becoming frustrating.
GUI, Commander interface
FTP / SFTP