To set speed limits use MaxDownloadSpeed and MaxUploadSpeed properties.
See example code:
Ftp ftp = new Ftp();
...
ftp.MaxUploadSpeed = 5; // limit upload bandwidth to 5 kilobytes per second
ftp.MaxDownloadSpeed = 5; // limit download bandwidth to 5 kilobytes per second
...
ftp.PutFile("file","file");
For unlimited speed set both properties back to zero.