If we could eliminate the possibility that servers on the receiving end are slow, or shaping ftp to whatever limit you are seeing, then there would be more of a chance at pinning the blame on WBS...
My suggestion to those who are experiencing ftp speed limits - if you have access to multiple servers - preferably on different continents, then attempt uploading different files to each of these servers - I say different bcos WBS' Poxy server probably caches uploaded via ftp files, also checkout incoming ftp speed limits...If you consistently get the same results no matter what server, and no matter what file, then there's more of a chance at pinning the blame on WBS...