Before connecting to the server, try enabling the UseLargeBuffers
option:
sftp.Options |= SftpOptions.UseLargeBuffers; //(C#)
sftp.Options = sftp.Options Or SftpOptions.UseLargeBuffers; ' VB.NET
sftp.Connect(...)
(where 'sftp' is an instance of the Sftp object)
Does that make a difference?