It's probably because your NAS isn't powerful enough to transcode (it is attempting to burn the subs into the image on the fly).
Convert your subs to an aTV natively supported format.
Just a small correction as I keep seeing this a lot. SRT subtitles are not being burn-in since PMS 0.9.7.27 Instead, they are being streamed via HLS (this still requires some horse power on your PMS). This is, at least, the case if you have compatible H.264 video stream inside your file.