I have a QNAP 259Pro+ set up as my Plex server. I've added by library and setup opensubtitles.org agent for my movies. Looks like this is working just fine. Subs are downloaded for my the language of my selection.
My Plex client is my Xbox One. What i see, is that when i stream a video from my NAS Plex server (whether in 1080p or 720p) playback is smooth. I actually didn't expect 1080p to be possible with my 259Pro+, but gladly it is.
However, as soon as i select the subtitle that was downloaded from Opensubtitles, the playback of any movie (1080p or 720p) will begin stuttering. For 1080p movies playback stops every 5-10 seconds, for 720p playback can go for maybe 30 sec without stopping.
If i then de-select the subtitle, playback will work as normal again.
Is there anything i can tweak for my setup, in order to have the subtitles to play nicely?
My guess would be that your NAS is not powerful enough to transcode the data for playback. All subtitles have to be burned into the video feed, which means when you have them enabled, you have to transcode.
You could easily test this by installing PMS on a more capable/powerful machine and running it from there. If it works normally, without issue, then you know that your hardware isn't powerful enough to make it work at the moment.
This might get solved in the future though as Microsoft adds support for media playback. But it also might not, only time will tell.
I can confirm that CUP utilization on my NAS increases dramatically when selecting a subtitle, so transcoding is indeed going on.
Is this a compatability isssue on the Xbox? Does it not support srt, since transcoding is needed?
Yes, same answer for both questions. Microsoft is supposed to be adding more support later on for media playback (DTS) and etc. But we don't know the full extent, so it is not certain whether support for SRT subtitles or other various codecs will happen at all.
Once Microsoft adds support for it, Plex can then update their app to make it work with it. Luckily transcoding works fairly well if you have the system for it. I'm loving having Plex on my system now, but hopefully everything will get added at some point, so it can be direct stream only.
