Sign up for Plex Pass so you can enable hardware transcoding. This puts the video transcoding on the embedded GPU, not the CPU.
An i5 7th or 8th gen would give you multiple x264/x265 1080p transcodes, including room for audio transcodes, which still hit the CPU. If you exceed the capability of the CPU, then drop in a Quadro P2000, for ~$400 USD, which can handle many more transcodes.
I’ve a Synology DS918+ with a Celeron J3455 CPU. It can handle a couple of 1080p transcodes w/o problem using h/w acceleration. No hard data, just me performing some informal testing.
I’ve a i7-4790K in my desktop. I’ve had five or six 1080p H.264 transcodes running with h/w acceleration. Again, informal testing. I had multiple browser windows open, each with a separate movie/transcode.
Also, see this post WRT P2000:
