That's where it starts to get a little more complicated as there are many factors in figuring out what potential playback issue is going on.
The majority of my playback is direct play meaning it doesn't transcode anything so virtually no load on the CPU and it basically is just seeking the file if I move ahead of rewind.
If it's transcoding, you have to get the data and let the CPU do its work and give you the data.
Each player for Plex is a bit different in how it buffers and how it works so there really isn't a one size fits all setup unfortunately.