What is my bottleneck?

I would say you have the optimum setup with the plex metadata on your server and fast disk. However, I wonder if the cache has become stale. There is a bug in the Cache/PhotoTranscoder which can grow unbounded. Take a look at this thread below as it might help with that issue…

Regarding …

… what is reported in your plex dashboard when you attempt to play a 4k movie ?