Unfotunately I don't think you understand.
Oke I understand that. But how is it possible that VLC Media player don't have any problem with that even on a slow laptop.
Because VLC is just deconding the video stream and compositing the subtitles over the video with the assitance of video decoding hardware (i.e you video card).
because the WD media player on his self can read it and mount it on the movie.
Yes it can if the video is played from local storage or a network share. Again the WD TV is just decoding the video and compositing the subtitles over the video with its onboard hardware.
I think it's only one simple command to take .srt files with it.
No it is not. For Twonky server to do this, it would have to demux the video file (into the elementary video and audio streams) and then remux the file to include the subtitles. This is not a simple process and can require a considerable amount of CPU performance. AND, there is no hardware performance assist like when the video is being displayed.
But the problem is still that the WD TV will not play external subtitles via DLNA. And most DLNA mediaplayers don't because there is no STANDARD method of doing it.