Pretty much, Unless the player can accept the subtitles AS-IS, PMS and playback suffer. The NAS boxes are brutalized by subtitle burning.
Text-based ASS & SSA should not be so hard for PMS to convert because, as you pointed out, it takes a few seconds manually.
This is on the To-Do list now.
The most painfully obvious subtitles are the image-based ones (PGS, VOBSUB, and DVDRIP)
These have always been a problem.
I recently found documentation which shows how to do image subtitle burning (PGS, VOBSUB, DVDRIP) in the hardware. It looks easy but turning out to not be as easy as it seems.
Between all the 
, I’ll get there. There is hope. No promises yet but hope.
Left alone long enough and I’ll get it done sooner ( “left alone?” hahahahaha)
As for making subtitle processing mult-threaded, The answer is regrettably no.
Subtitles must be burned into each video frame as they come out of the HW decode stage but before HW encode (to send out)
