The DS920+ uses the Celeron J4125 processor, which has four cores.
The Resource Monitor in DSM shows the total utilization for all four cores.
Subtitle burning is single threaded, meaning it uses just one core.
100% of one core shows up at 25% in Resource Monitor.
Were you to SSH into the NAS and use top or htop, you would likely see one core near 100% and the other cores each at a lower level. Add up the utilization for each core, divide by four, and you’ll end up in the 30% - 40% range.
The video transcoding is using the onboard GPU, not the CPU. GPU utilization is not reported in Resource Monitor, so it is not part of the 30% - 40%. I’m not aware of a way to view GPU utilization on a Synology NAS.
Here’s a post with details on how the Plex transcoder handles subtitle burning. Ignore the part about video drivers, it is not related to subtitle burning:
