Hi, I have a 100/100Mb internet access. Using av USG 100 as firewall. Using Speedtest.net I only get 20/34 Mb, and using iTunes "rent movie" i not working, and internet is generally slow. Connecting directly to the WAN from a PC I get 95/95mb on Speedtest.net. Disconnecting all LAN/DMZ from the USG 100, and using the PC directly to the LAN i get the same result 20/34 Mb.
Do you have antivirus or intrusion detection or other USG filters turned on (paid subscriptions)? These significantly degrade bandwidth, to the kind of levels you report.
I don't know anything about iTunes movie rental, but maybe it tries to use UPNP, which the USGs don't support. You would probably have to open a port for it, and maybe set the iTunes preferences for the same port number. A search on iTunes and ports might yield some clues.
This design works well and the software is great. However the hardware really needs an update. It might have been adequate 4 or 5 years ago but is substandard for the increasing throughput of modern systems