Iirc, you should just leave it on defaults.
If you are cpu bound, none of them have much of an effect on reducing your buffering.
Since you have plex pass, you should invest in a gpu that is new enough to support the codecs you are using/transcoding.
If you are transcoding 4K content, you essentially need a gtx 1050 or newer, or a 7th gen intel cpu with 600 series igpu.
See also @ [INFO] Plex, 4k, transcoding, and you - aka the rules of 4k
If you are not using 4K, perhaps some older gpu may help to varying degrees.
