OptiPlex will have the grunt. I rolled out a bunch of Precision 3630’s with i3-9100’s and the 3630 chassis has lots of drive bays to roll in local storage.
…so digging in I see several alarms.
I’m curious what you mean by fixing their client settings? Is there something that should be set on the client-side?
YES. If a client is not set to highest/maximium/directplay, it can attempt to transcode. Each client must be individually configured, on the client device. Direct streams require little cpu, but as soon as a client requests to transcode, it can wreak havok on an underequipped cpu (like your 4xxx).
I would love an option in plex, server-side, to block transcoding from the server, if nothing else to prevent errors dragging server down. It would be nice… to have on a per library basis, @ChuckPa, to apply to a 4K HDR, library, but not a 720/1080P library 
nudge
@sim
Going back to this thread/post you linked:
ChuckPA explains all.
Your media is in HEVC, your client can’t process HEVC, your cpu is being dragged through the mud trasncoding HEVC in realtime and has no gpu assistance.
no cost:
- insure your media is in a format that your clients can direct stream.
- insure your clients are set to direct stream/play, maximum quality.
- avoid PGS subtitles as that can force a transcode
low cost:
Super new clients. AppleTV 4/5 with enhanced player supports virtually all formats and can handle PGS subtitles without transcodes, but that’s exception and not the rule.
more cost:
Intel cpu 7xxx and newer can transcode on gpu all the popular formats HEVC10 etc on the igpu without dragging cpu into the mud.
