Plex has said they are working on using the GPU to burn subtitles. They’ve mentioned possible delivery, at least in a forum preview, in 1Q2024 (reference). I doubt you’ll see more definitive information (dates, PMS version, etc.). As you noted, Plex does not publish such information.
Current subtitle burning is performed on a frame by frame basis: Decode the frame, add the subtitle to the video frame, encode the frame. Decoding & encoding can be performed by the GPU, but adding the subtitle to the video frame is performed by the CPU.
As far as upgrading the system, given how things currently work, you would need a CPU with faster cores. Unfortunately, I’ve not seen any information regarding what CPU is needed to burn subtitles into 4K media.
Personally, I’m holding off any upgrades until we have an idea of what Plex will deliver and its requirements. In the mean time I’ll proceed like I always have: local 4K HDR playback is direct played (Shield, FireStick 4K), and I don’t share 4K remotely, so no need to worry about transcoding & burning subtitles.
