If you transfer files via SFTP then we recommend the following option to increase the transfer velocity:
load_class_name = com.sos.VirtualFileSystem.SFTP.SOSVfsSFtpJCraft
In this case another class (JCraft instead of Trilead) is used which is more faster.
Furthermore the transfer velocity can depends on the option buffer_size (default= 32000).
Try
buffer_size = 16000
or even less.