Hi,
While my current system ( with Atom C2751 ) is good enough for keeping storage and playing 1080p x264 video via Plex, I see some problems when playing 1080p HEVC MKV files. I think I need to upgrade CPU for that. What do you recommend?
From my experience, Plex requires a cpu with PassMark = 2000 for each full HD stream that is transcoded from h.264 and about PassMark = 5000 for each full HD stream transcoded from h.265 (HEVC.) So the answer depends upon how many streams you need to concurrently transcode.
If you have playback devices that support h.265, such as many newer TV's and Roku's, then Plex will not have to transcode when you playback to those devices.
So the answer really is... it depends upon how many streams you need to transcode. If you use playback devices that support h.265 then you will not need to transcode when playing on those systems and the CPU of your system won't matter. For playback on devices that do not support h.265, then it will be determined by how many of those devices you are playing to. Make sense?
It becomes a little more complicated when subtitles are involved - and for that, I would refer to the Plex forums.