AppleInsider today is reporting on a potential cause for slow Apple TV and iTunes download speeds: the use of Google's DNS servers. Last year Google started a public DNS service, promising faster speeds and security, but AppleInsider mentions that because some download services use the DNS server to locate the nearest download server, using Google's DNS servers (which are at one location) will route everyone to the download server closest to them and bog it down.
If you are experiencing slow network performance with iTunes you can check for alternate DNS settings on your system by going to the Network system preferences, select each network connection individually, and click the "Advanced" button for that connection. When the advanced window pops up, choose the "DNS" tab and remove any servers listed that are not grayed out. If you use an Apple TV, go to the settings and choose your network connection (Ethernet or wireless), from where you can edit the DNS settings.
The Google DNS servers are "184.108.40.206" and "220.127.116.11" and while removing them may help, if you have other public DNS options such as OpenDNS or UltraDNS servers, then you might also try removing them and rely on your local ISP-supplied DNS servers, which should be supplied automatically, to see if network performance increases.
Keep in mind that if you use a router for your network connection you will need to log in to its administrative settings to remove custom DNS entries. Computer systems on the local network will use the DNS entries in the router, but may only show the router's IP address as the DNS server it is using.
Using custom DNS servers may still be desired, but unfortunately there is no way to prevent certain services or applications from using a given DNS server. One way around this might be to create a specific network location in the network system preferences (using the menu at the top of the window) and save one with the desired DNS servers and one without. When multiple network locations are configured, the system will add a "Location" submenu to the Apple menu, which can be used to quickly switch to the faster one for iTunes downloads.