Replying to keep this thread alive as I’m having the same issue.
I migrated my Plex library from a Terramaster (TOS5) setup to UnRAID and everything works, including HW transcoding via Intel-GPU-TOP and opening access via /dev/dri. If I turn on HDR Tone Mapping though, it doesn’t use the iGPU and buffers like no other.
It worked perfectly fine with TOS5 so I’m not sure what the issue is.
There was a reddit thread where someone had the same issue and ended up reinstalling Plex from scratch which seemed to fix the issue. I have spent a lot of time on metadata customization (i.e., posters, collections, etc.) though and would prefer not to go this route unless absolutely necessary.