I was referred to this site by Royal TSX customer support. They are using the Rebex libraries for FTP.
I experience very slow up and download speeds when using SSL, I think the slowest one being the explicit option. This is only on macOS, not on Windows, where speed is just fine.
Without SSL the Mac is downloading at 50MB/sec, with explicit SSL, downloading is going a only 1 to 1.5 MB/sec.
Because I thought the problem seemed to be platform related, and the people from Royal TSX are just using your libraries, I decided to test for myself. I downloaded Visual Studio for Mac, downloaded a demo of your library, opened the project "FtpGetPut_CS" which is in the samples folder and ran it from the IDE with the external console so I could enter a login and password.
Results were as I feared. With the "none" option, the download is much much faster. It seems to me there is something wrong with your implementation on macOS, when using FTP over an SSL connection.
Can you please look into this?