Hello,
I'm also experiencing very slow SFTP download speeds compared to those I reach with other clients. While WinSCP reaches 15 to 16 MBit and Beyond Compare 4 (a diff tool with FTP/SFTP capabilities) about 12 MBit, Directory Opus Pro 11.10.1 Beta x64 (on a Windows 7 x64 system) with Advanced FTP feature gets stuck at about 1.3 MBit when downloading a big file (I seldom transfer small files from or to a server, I mostly use archive files to speed up things).
I have a German VDSL connection with 25 MBit downstream and 5 MBit upstream. Using FTP, the maximum speed for downstream and upstream is almost reached; with SFTP, DOpus' upload speed lies around 3 MBit, which isn't brilliant (WinSCP as well as Beyond Compare both almost reach the maximum speed of 5 MBit), but I could live with it if necessary. But 1.3 MBit download speed really hurt.
In an older thread Greg asked a user to send a mail with the details. I set up a test account for you (maybe this is related to just some servers?) - is it okay if I send you the credentials so you can test it for yourself as long as you like? I really love Directory Opus and I'm using it all the time, and I don't like opening additional software for data transfers from a server. But being a web developer, I have to do this very often, and "unsecure" FTP is not an option.
In the old thread I linked to (it's closed now), leo said that there might be long-term changes regarding the FTP feature, but that was in April 2013... Is this still in the works?
Thanks in advance for any help! 
Cheers,
Jan