I myself just finally figured out after many years that my brutally slow connections to ftp/sftp's were due to DOPUS. There is also another thread on the subject here: SFTP too slow
For me if I use DOPUS I get download speeds maxing out at 150KB/s, if I use WinSCP I can get 3MB/s from the same server both using the sftp protocol. I almost feel like a fool for not realizing this years ago, but I always blamed my old web host.