Im a ITer as proffesion. And I can tell you. the connection depends on the follow things:
1. Internet speed
2. You can have 100MB/s but providers always short the connection speed, you can do a speedtest on speedtest.net
3. Depends on your network card, every modern network card supports at least 100 MB/s
4. it is depending on your FTP server (probberly there should be some options whitch says how much internetpackages (speed) a user is allow to use for the FTP connection, this option is normal because if alot of people got a FTP connection to your server and they all got the possibility to use 10MB for the FTP connection, you don't have any speed anymore with 10 connections.
5. Firewall, if the signal have to go true alot of firewalls it will slowdown the speed. Because the signal have to be checked by a few firewalls. Of course this takes time
6. The Upload/Download speed from the guy who is connecting to your server.
EDIT: it also depends on the distance, if someone connect to a FTP server in Norway and you are from russia the signal have to be travel around a lot of differend providers and firewalls
Think thats it.