Spoke with AppleCare... they're stumped. =P
I have full bandwidth (25mbps downstream) Ethernet hooked right up to Apple TV 2. I have no firewall configured. All other content works flawlessly: YouTube, AirPlay, Netflix, Home Sharing, etc. iTunes Store seems affected by peak hours (which doesn't make sense). I've hit the Genius Bar and replaced ATV2, no difference. I've called AppleCare, they're stumped. I've contacted iTunes Store support (at the suggestion of AppleCare), so far they've only thrown links at me as suggestions, all of which I've already tried.
I just tested and downloaded a TV show straight to my iTunes library on my desktop... at 1MB/sec. That's less than a THIRD of what my connection is capable... and that was directly to my PC.
I'm more and more beginning to think that this whole problem is on iTunes Store server side. Perhaps just in my local area? I don't see how, though, when most everyone else seems to be getting acceptable performance. I don't need FULL SPEED, I just need ACCEPTABLE speed. I'm sorry, but waiting 2 hours for a 45 minute show is UNACCEPTABLE.