It can burn at least 1 subtitle (srt) if you are using hardware encoding for the video. My TS-453Be (also a J3455) with Plex Pass has done 7 streams one doing subtitles, 4 video trans-coding and several more audio trans-coding, running at 85-ish percent. But yes subtitles puts a hurt on the CPU. Without the subtitles I have seen it run 12 streams and still only be at 70-ish percent with various (I think 6) video encodings.
In my opinion the lifetime Plex Pass is worth it for HW transcoding if one plans on using Plex and supporting servers for any serious length of time.
See below (notice I also have A LOT of different devices using it - Sony TV, Samsung TV, IPAD, Plex Web, 2 Samsung android phones one an S8+ the other an S9 and a Roku). This was all on 2/26/21 Plex FW 1.21.2.3943, QNAP FW 4.5.1.1495 :
