Experiencing the same YouTube app problems with the Samsung UE40EH5300 Smart tv. Videos play fine for about a minute, then they start to stick. Two minutes in, the video freezes completely. I don't think it's an internet speed issue as Youtube plays fine on my laptop, plus the Vimeo tv app plays HD videos without any problems.
I hope this can be fixed as YouTube was one of the main reasons I went for a smart tv.