The plex app is slow compared to my phone / web, but it's on a par with the rest of my samsung hub apps.
I'd much rather wait the few extra seconds it takes to respond, than to have to use another device to control it (after buying an extra device to interface). Pulling my phone out, unlocking/swiping, opening app, etc etc is a total chore for couch times. I can do everything on my TV with a single remote control. I have a tablet that could do it, but again it's a chore if I'm lying down. One hand in my pants, the other cantilevering the remote control that is focussed on the tv. Good times.