@ChuckPa, thank you for providing additional info! Now it makes sense that subtitle burning consumes an entire CPU core and therefore results in 25% utilization rate.
On the burning side though, I can confirm that Plex is able to burn subtitles with the configurations I mentioned above without any buffering. I’ve already shared pictures, but can add logs if needed or a video with before and after results.
About 2 weeks ago I got frustrated that I couldn’t turn on subtitles (image and text) without disabling hardware transcoding and started researching a lot. As of today, I can enable image-based subtitles and utilize hardware transcoding for video and audio and one CPU core for burning subtitles. No buffering. I tested with 2 simultaneous transcodes, it was smooth with 0 glitches. I was seeing some short buffering when testing with 3 simultaneous transcodes but I could still watch it because it was less than a second every 10 seconds.
I always do preprocessing by removing unneeded tracks and subtitles, but some Plex clients will always burn subtitles even if the setting is set to Never. One example of this is TV clients. Most of them will burn subtitles (even text-based) if any transcoding is happening (video/audio/re-boxing or when simply selecting the second audio track.)
I appreciate all the effort to bring HW to burn subtitles. I believe it will be a huge win for Plex since many users don’t want to spend over 3k on decent hardware just to stop worrying about subtitles.
To sum it up, my NAS has always been able to burn subtitles and play 1080p video without lag when I disable HW transcoding, but my CPU would be going between 90% and 100%. When I was turning on subs, the video would become unwatchable due to constant buffering with HW transcoding enabled. Now with this fix, I can utilize HW transcoding for video, and burn subtitles, all while having no lag and keeping CPU at 25% (as you said, one core at 100%). I don’t know the technicalities of why this is “working”, but at least it gave me the result I was looking for.
