You know it's bad when your kids give you that look: "Dad, what have you done to the network this time?!". My wife is a saint and takes it all in stride, but the kids haven't yet reached that level of home-networking nirvana.
The family frequently sits down together to watch a few YouTube videos on the AppleTV (3rd gen). It's fun for all of us, the oldsters get to keep up with the internet memes and the kids get to practice their acting skills when looking shocked by some of the videos.
About 4 weeks ago, though, it wasn't a fun experience. There were huge lags before the videos began to play and they would typically pause every 15-20 seconds waiting for more download. Oddly, movies from Apple seemed just fine. Our computers reported 25+ Mbps download speeds. How could this be? Must be some blowback of the great Apple/Google wars, (of course mainly Google's fault, no doubt! :)
I finally sat down and researched problems with AppleTV and YouTube content. There's a lot of noise on this topic going back a few years, but I think most of it was just that...noise. Finally, I came across some informative articles describing DNS issues that can arise when using alternate DNS providers (like OpenDNS) while accessing content that is provided by big Content Delivery Networks (CDNs), think Akamai and Google for example. I had recently switched from my ISP's recommended DNS server to OpenDNS (with a DynDNS backup) so this seemed worth pursuing. I still believe OpenDNS provides a valuable service, but this problem might be a sticking point for some situations.
I ran namebench to test my DNS settings and possibly make some other recommendations. To my surprise, my ISP's DNS server responded faster than any of the other DNS servers that namebench tested, including OpenDNS and DynDNS. Seemed promising, but would this resolve the AppleTV/YouTube problems.
I ran the Mountain Lion Server App to adjust the DNS server settings and also change the appropriate DHCP settings (both of these services are provided by Mountain Lion Server, not by my ISP's router...). Zip, boom, voila! Back to super-fast YouTube videos on the AppleTV.