A lot of anecdotes regarding slow iTunes and Apple TV downloads are spreading around the internet, and it turns out third party DNS—like previously mentioned OpenDNS or previously mentioned Google DNS—might be the problem.
If you're experiencing slowdowns when downloading large files from iTunes or your Apple TV (or, in some cases, other places 'round the net, like YouTube), the problem may be your DNS. AppleInsider explains it best:
Network users are supposed to look up IP addresses from a nearby server, which itself queries and caches answers to name and address lookups from other DNS systems, distributing the workload across the network. CDNs like Akamai, which Apple works with to deliver iTunes downloads, use DNS lookup information to locate where users are, and then optimise content delivery via the nearest available server.
When millions of users all tap into the same DNS server addresses to resolve domain names, as Google DNS does by design, Akamai and other CDNs route content to those users along the same path, preventing the network from working optimally. This causes problems not only for Apple's iTunes, but also any other media streaming or download service that uses a similar CDN strategy to distribute downloads.
So, while Google DNS or OpenDNS might have other benefits, they could be causing some pretty major slowdowns in other areas. I haven't experienced this myself, since previously mentioned Namebench already discovered that my ISPs DNS was faster than Google or OpenDNS anyways, but it may be something to look into if you're using a third-party DNS server and you're having problems (if you don't use iTunes and aren't noticing other slowdowns, you could be fine). Hit the link to read more. I'd also recommend reading AppleInsider's piece on it, since it's the most informative I've read on the subject so far.
Update: Laura Oppenheimer of OpenDNS wrote in to let us konw that, while Akamai may still cause some suboptimal performance (more outside out the US than anything), OpenDNS has arrangements with a number of CDNs that make this mostly a non-issue. If you're in North America and using OpenDNS, you probably won't experience the above problem.
Apple TV, iTunes Downloads Slowed by Google DNS | AppleInsider