@adefaria Based on the Plex Dashboard screenshot in your first post, you are not using hardware accelerated transcoding. If you were, you would see (hw) in the video transcode section. Transcoding using the CPU, instead of the GPU, will also make CPU utilization spike, no matter what is happening with subtitles.
In Settings → Transcoder, make sure you have:
-
Disable video stream transcoding= NOT selected. -
Use hardware acceleration when available= selected -
Use hardware-accelerated video encoding= selected
Also, review Synology FAQ 28. Using Docker for HW trancode & Tone Mapping - How To.
Make sure Plex Docker can access the GPU in the Celeron processor: --device=/dev/dri:/dev/dri \.
Reviewing your prior conversation with @ChuckPA may also help.
