That's interesting. So the one I bought in Nov 2013 would hang up at least once on every movie played in Plex - which was such a shame because it is such a beautiful platform - now I rely solely on iTunes for movies streaming (and old games) - I basically have to convert them all. Major Pita.
Netflix was just super slow as was Prime for me on the old one, the new one is a year newer and it started crashing almost every time I played something back in Netflix and Amazon. I just gave up.
I wanted some feature where I could go back and shut down processes - it just seemed like it was overwhelmed.
Fwiw, it sounds like very similar usage - for me the speed is definitely noticeable on both the xbox and apple tv compared to the others, even baring the crashes that was enough for me. The bad thing is, I don't have access to plex (hopefully the new Apple TV gets an App) outside of the Samsung, and I'm just not willing to try at this point because itunes is serving the need.
I have a 2013 LG Smart TV, and Plex runs better on there than any other devices (I have Roku 3, Apple TV, and Chromecast). However, it might just be the location to the router (right next to it).
That being said, Plex is amazing, and if you use it, can eliminate almost about any need for cable. No conversion, metadata is automatically imported, can download captions automatically, and awesome interface. I setup utorrent to manage via mobile, so I download what I want away from home and its on Plex when I get back. Through devices and password sharing, I have been able to get any programming without dreaded comcast, save for CBS Sports.