I had similar problems, it always lags. I solved it with a hard reset of the TV. try to restore the TV to factory settings, if the problem persists call the Samsung call center, they can remotely perform a deep reset that solved the problem for me.
Nope, everything i watch just comes up with a spinning circle about half an hour in, hangs for about 20 seconds and then crashes out, however if i press forward when the circle appears quick enough then it will not crash but later on it will happen again.
I installed emby server and nothing like that happens so its not anything related to the server or network, must be something specific to plex.