hi
what is your topology?
COMCAST connected directly to WAN/Internet network and your "client" doing the download on the LAN/Access Network?
are you doing this wired on the network or wireless using a client and an AP?
What is your VSC config? Are you using Bandwidth control or have the Firewall enabled? If you are wired then your probably going through the Default/First VSC.
Just to test, you can disable Use controller for: Access Control in your VSC, then go to Controller -> Public Access and Disable Access Control. then redo your throughput test. tell me if it works better
thanks