Hi there

If you don't actually need to access your Multi-media files from OUTSIDE your LAN specifically with a Media server then using the VLC version bundled into the Amazon Firebox (note not the firestick as it's much less powerful) gives a far better performance - especially if you have your files on relatively low powered NAS systems.

For starters the amount of transcoding is very much reduced - VLC plays .AVI and H265 without the need for transcoding.
Another one is that in a lot of cases the subtitles can be dynamically fetched when using VLC and if you have separate files it can load those too.

PLEX - don't get me wrong - is an excellent multi-media server but if you have an amazon firebox then it's a much better bet.

Of course if you need to stream the stuff to different TV's without a firebox then PLEX is great and the tool for the job.
While only using say your main TV IMO you'll get a much better bet using VLC with the firebox -- it's quite a powerful processor and will easily keep up with even slowish LAN speeds over wifi and it can also do a reasonable amount of "pre-buffering" so you'll get streamless playpack (no jittering etc) even when the media is on external USB drives attached to your NAS.

Remember also that PLEX currently doesn't seem to offer any UHD (4k and eventually 8k) streaming. VLC will handle it or at least the 4k stuff -- don't have the luxury yet of an 8K TV or even any 8K media !!!.

Note for 4K video use HEVC/H.265 rather than H.264 -- much smaller file sizes too -- but you do need a bit of CPU power for this - some NAS servers might give up the ghost here when transcoding if you use servers that require transcoding !!.