the raw throughput or bandwidth to the drive, is much less important than the latency.
another good thread @ theoretically speaking, how large of a library or libraries too large for plex to handle?
@erik.zeitz above you indicate running ubuntu, open an HTOP window on the vm that plex runs on and monitor the threads.
watch for red D’s in the the status (S) column, that means that vm does not have enough IO. an occasional few short lived D’s are not a big deal, but if there are a lot or they are showing D for long periods of time, then you are definitely seeing insufficient IO.
