In case you use Rebex FTP or Rebex FTP/SSL:
Yes, setting Ftp object's UploadBufferLength to a larger value can result in increased performance in many cases. Try setting it to something like 64KB and let us know whether it helps.
Another trick FileZilla does is enabling larger TCP window size (it sets TCP send buffer size to 256KB and TCP receive buffer size to 4MB). However, we were quite reluctant to do this in Rebex FTP because this has been observed to only increase the speed in some scenarios and needs quite a lot of memory. However, if using a larger UploadBufferLength doesn't help, we are ready to work with you to give this a try as well.
In case you use Rebex SFTP:
Before connecting, enable Sftp object's UseLargeBuffers option:
sftp.Options |= SftpOptions.UseLargeBuffers; //(C#)
sftp.Options = sftp.Options Or SftpOptions.UseLargeBuffers; ' VB.NET
sftp' is an instance of Rebex.Net.Sftp)
This enables several FileZilla-like tricks (like larger TCP window size and some internal received data buffers).
FtpOptions.UseLargeBuffers is now available in Rebex FTP(/SSL) v3.0.3793.0 as well.