Doing some additional testing on my current system – the VOBSUB burn has a huge impact on CPU usage, far beyond what I’d expected.
This other box is running an unlocked 1080Ti with a Threadripper 1950X (HW acceleration enabled in Plex) – the subtitle burn doesn’t seem to impact the work the GPU is doing at all, but is adding a ton of work to the CPU (up to 80% utilization @ 10 transcodes). I don’t fully understand why that’s the case.
EDIT: Further research on the Plex forums shows that image-based sub encoding is a single-threaded process that occurs on the CPU. So that explains it.
Seems the smart move would be to scrub my library to replace all image-based subtitles with SRT alternatives (which does not sound fun); or do something like add a P2200 to my Proliant DL380 Gen 8 instead of using the HP 290 (so there’s CPU power available, should it be needed).
EDIT: After further reflection, the HP 290 doesn’t meet my needs (family Plex server, 1080p h265/h264, various subtitles and non, up to ten streams). Nothing wrong with the box per se; I think I misunderstood its capabilities. Probably around 20+ 1080p h264 or 12+ 1080 h265, so long as no image-based subtitles need to be burned – which drops the capability significantly. Definitely an edge case, but just leaving this here for anyone else who is in a similar situation.