We managed to get zap time < 300ms on our IPTV platform, but that required A LOT of additional work configuring encoders, networking equipment and messing around with STB firmwares.
There's bunch of places between the video source and you that add their own buffers (not to mention the actual I-frame issues). Usually the most critical are encoders themselves (need to generate closed GOPs and a lot of commercial expensive hardware stuff doesn't give you enough control) and the player software on STBs (those usually have large buffers pre-set).
In the end it's rather large amount of work most IPTV (and digital cable) providers don't care about because... hey... where are you gonna go if you're unhappy with their service? :)
There's bunch of places between the video source and you that add their own buffers (not to mention the actual I-frame issues). Usually the most critical are encoders themselves (need to generate closed GOPs and a lot of commercial expensive hardware stuff doesn't give you enough control) and the player software on STBs (those usually have large buffers pre-set).
In the end it's rather large amount of work most IPTV (and digital cable) providers don't care about because... hey... where are you gonna go if you're unhappy with their service? :)