I use Plex on the FX160 and basically nothing needs to be transcoded. Streams fine to my PS4, Chromecast and iOS devices. The content I have is in H264 though, I don't leech/encode any H265 yet.
I'm guessing that the Atom processor in that device can't handle H.265->H.264. I'm using a two-year-old i5 (top-end of that class of processor at the time) and while it manages, it's pretty close to the edge with the version of ffmpeg they're using.