Why can't iTunes movies and TV programs stream without buffering over VPN?
On an Apple TV4 connected to 1 Gbps FTTH, Netflix, Hulu, Showtime, BBC iPlayer and more all stream perfectly without buffering over a VPN from Japan to London or the US East Coast. The exception is Apple's iTunes movies and trailers and TV programs which suffer from AWFUL buffering.
Clearly Netflix et al. know a lot more about buffering technology than Apple (which should takes some tips from Netflix—or better still buy Netflix).