@Val: Check your upload dossier. Merci!
@all: thanks to yannik.val I could test some issues in a real situation.
Downloads from my site are running fine, and as indicated by HFS, with the theoretical maximum speed of my upload capacity of 70kB/s : 512kbps.
But when I upload files, the bigger the files are, the longer it takes before the actual sending begins and the upload speed, as indicated by my system, is only half of the theoretical rate: 45kB/s.
Additionallly, it appears as if the sending takes a lot of system recources: the progress window can't be refreshed and appears only after the upload of all selected files has been finished.

(Of course, there were no other connections open.)
This happens too, when testing locally 127.0.01 or via LAN 10.0.x.x or 192.168.x.x on my machine. In this case I'm sending and receiving on the same machine and that would explain the high load)
When val's downloads were ongoing, I tested the progress window. My (local) connection did not display the ongoing downloading files from the other user, which is OK wrt. privacy. Can you confirm?