wdtvlive client, synology server - subtitles without transcoding?

I’m using a wdtvlive box as the client, it can handle a bunch of different server types natively. My server is a synology 1511 NAS, just installed plex a few days ago. Everything seems to be working fine, except for subs. When I turn on forced subtitles, every video takes about five minutes to load because it can’t display subs without transcoding. It’s not a super fast processor, so transcoding is a lot of work for it. Is there any way I can display subtitles without transcoding? Or even just reduce the amount of work the transcoder has to do so that it’s just overlaying the subs and not trying to recode the whole stream? It’s important, as I’ve got one deaf and one hard of hearing family member, so we need subtitles on everything

Depending on exactly which WD you have, pointing the WD directly at a shared folder on your server might be more effective than going through Plex.