Not surprised about nothing from the Ubuntu forums, it's more of a basic network problem (that's not really a problem).
I'm guessing your home network is something like:
fromISP to a cablemodem to a router/firewall to a switch and all home computers are plugged into the switch.
Traffic from A on the home network to the Ubuntu server goes through the switch, does not hit the ISP network at all so will have the full bandwidth available (if the switch, A and the Server have GigE network interfaces, they have Gig Bandwidth available. If they all have 100Mbps interfaces, they have 100M available).
From the outside world traffic is fromInternet to ISP to cablemodem to router/firewall to switch to server, from server to switch to router/firewall to cablemodem to ISP to Internet.
Residential broadband is typically capped so ISP to your house is slower than theoretical bandwidth, so you start out with requests hitting your server slower.
Residential broadband is usually asymmetrical (except FIOS maybe) so bandwidth from your server back to the outside world is slower than theoretical. Something like Comcast 50/10 means that the max going into your network is 50Mbps, the most coming out of your network is 10Mbps (that is theoretical max, actual max is likely lower by a few percent).
So what you are seeing is not at all surprising, especially for residential service. You also need to keep in mind units of speed. Most network interfaces talk about "bits per second" while other things report in "bytes per second" (factor of 8 right there).
Could the ISP be throttling traffic? Sure, their network, their rules. Is it likely? Offhand I'd say no. I think your simply seeing the effect of asymmetrical broadband link.
Is there "...a setting I need to change..." Again, probably not.
Full bandwidth on your home network is drastically different than "full bandwidth" once you hit the ISP network (ISP network starts at the broadband interface device which is the cable modem or the FIOS box or the DSL interface. You may own that device, but hooking it up to the ISP network gives them permission to control it which includes configuring it and perhaps setting ratelimiting in it).
Bonus Security Soapbox:
From a network security aspect you really shouldn't be exposing an FTP server (please tell me it's not an anonymous FTP server) to the Internet at large unless you really have it isolated from the rest of your private network. DMZs, routers, firewalls, tight access lists, etc. You'd be much better off configuring an inbound VPN to your home network (or just the server) so that at least you start off from a much more secure place. At a minimum, use sftp instead of regular ftp (sftp is the ssh equivalent so it's more secure). But, your network, your data; I'm simply making a suggestion.