I'm having the same issue.. I have an apple tv 3 I just bought. It doesn't matter if I connect it wirelessly through my airport express, or wired directly to the airport express LAN port... Hulu, youtube, etc. all work fine... Just itunes content. It doesn't matter if it's via home sharing or standard movies\etc. a 45min tv show takes 40+ minutes to buffer!! a 2 hour move? 2+ hours.... My internet is comcast 25mb\s, wireless is Wireless N, and even with ethernet... this doesn't make any sense...
I've tried restoring the device, resetting the device, running network test, changing DNS, wired, wirelessly... Nothing matters... I just canceled cable and bought all my shows on itunes.. heavily regretting that decision. I come home from work, sit down to relax, and have to wait an hour before I can watch tv???
Funny someone mentioned hunger games.. We tried to rent that movie too as our first through apple tv... 930 at night, kid sleeping, and says we needed to wait 2.5 hours to watch it... we just switched on the xbox (connected wirelessly btw), and were watching it in HD within 3 minutes... got my money back from apple. dissapointing though, movies are cheaper. Plus why doesn't home sharing even work? I'm streaming from a 2011 imac I7 connected wirelessly at 300mb\s with no other traffic on that computer.. Very close to router and great connection... Really sounds like an ATV issue...
Working in enterprise level IT for the last 15 years of my life, and previously being an electrical engineer... doesn't make sense to me :-p fix the firmware already... almost like it's downloading slow on purpose to save\reduce bandwidth for... ?