If you transfer files via SFTP then we recommend the following option to increase the transfer speed:
load_class_name = com.sos.VirtualFileSystem.SFTP.SOSVfsSFtpJCraft
Due to this setting a different base class (JCraft instead of Trilead) is used which is faster.
Furthermore the transfer speed will depend on the option buffer_size (default= 32000).
Try
buffer_size = 16000
or less.