I think it's not a frame rate issue. I think the most important thing with 4K =Me to, something with framerate perhaps?
Do you use an external player like VLC or the built in player when you watch VOD?I have the same issue with 4K buffering, it works great when i use Netflix or 4K material on Youtube. However 4K VOD IPTV is constantly buffering. I have a ethernet connected 500 mb conection and use Nvidia Shield with IPTV smarters.