Frustrated by slow movie rental downloads via iTunes or your Apple TV? As it turns out, it may not be your broadband connection at fault, but rather a free domain name service (DNS) such as Google DNS or OpenDNS.

AppleInsider is reporting on a new quirk in the free centralized Domain Name Service (DNS) that many of us are using with our broadband connections. These services, which include free offerings such as Google DNS and OpenDNS, enable “client computers to look up IP addresses of Internet servers from their easier to remember domain name.”

The advantage to using a DNS offering that’s not native to your own Internet service provider is usually speed -- not only do services like Google DNS bypass domain blocking rules established by your ISP, they “can also speed network performance in some cases by resolving IP addresses faster.”

However, there appears to be one fairly large exception to that rule, which is a service such as iTunes. Apple uses the Akamai Content Delivery Network (CDN) to serve its media downloads, which is not designed for a lot of users to access it from the same DNS server address at once.

“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,” AppleInsider reveals. “CDNs like Akamai, which Apple works with to deliver iTunes downloads, use DNS lookup information to locate where users are, and then optimize content delivery via the nearest available server.”

The problem with Google DNS and other such services is that potentially millions of users are tapping into these CDNs at once, effectively slowing down service for everyone using the same DNS. This headache doesn’t strictly affect iTunes or Apple TV, but any service using a CDN in this manner.

AppleInsider recommends that users with slow media downloads “revert back to the original DNS settings,” and provides a link to Google’s own instructions on using Google DNS which can also help go in the other direction.

Update: Laura Oppenheimer from OpenDNS reached out to us after reading this post to clarify that the DNS issue doesn’t really affect their service thanks to agreements with many CDNs, at least for customers in North America.

“OpenDNS has arrangements with a number of CDNs that make this a non-issue for the vast majority of OpenDNS + Apple TV users,” Oppenheimer explained. “That said, with Akamai, especially internationally, it's still suboptimal. It's entirely workable, but not as optimal as it could be.

“In general, North America isn't really an issue since we have a sufficiently dense network topology. We’re very open to working to improve end-user CDN routing with Akamai, just as we have with other large CDNs,” she concludes. Good news for OpenDNS users, and thanks to Laura for bringing it to our attention.

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