https://www.cpubenchmark.net/cpu.php?cpu=AMD+FX-6300+Six-Core&id=1781
Well… Plex says you can do 2 - Plex may lie…lol:
https://support.plex.tv/articles/201774043-what-kind-of-cpu-do-i-need-for-my-server/#:~:text=If%20you%20want%20very%20basic,Intel%20Core%20i5%203.0GHz
Transcoding 4K video and audio while burning in subtitles is a full time job, IMO. You may have reached your ceiling - and then some.
Try to get something, at least, to Direct Play. Go get some srt subs: https://subscene.com/ (I have good luck there), or convert an audio stream to something that will Direct Play on your Device - or both.
View the Dashboard with Details, see what’s transcoding and see if you can do something about some of it.
Make sure the Plex App in the Device is set for Max/Original Quality.
Lighten the load until something good happens.
Or get more horsepower.
I have an AMD FX 8350, Plex Pass, and an AMD R9 380 Graphics Card Plex makes good use of. If you have something in that area, a Plex Pass subscription may be cheaper than an overhaul - if you know what I mean?
What else… let’s see:
If you have a Device of some kind - Roku Ultra/FireTV 4K Something/Other 4K something - you may have better luck getting that to Direct Play something that your TV Won’t.
Also, if the server is close to the TV and you happen to have an HDMI port in the Server, run a cable from it to the TV and Drag one of these BassAss Players over there - they’ll Direct Play most anything you throw at 'em.
Plex Media Player
Plex for Whatever
Apps/Devices links at page bottom.
Yea, that’s extreme, but don’t think for a minute I don’t have that option - right now.
… and read @TeknoJunky 's 4K Transcoding Bible - he holds mass every day at about this time… fire up the incense…lol