Then why the difference of getting a 1080ti?There should be no need for transcoding, it should just direct play.
Most likely you've got an image file format subtitle....OR the audio track is being transcoded causing kak.
I can Direct Play remuxed 4K stuff from a Microserver without issue.
If the Synology can be had cheaply off a forum member and will only do 1080p max then it will do the job.
But a multiple drive Microsoft with Unraid is a much better solution for future forward planning.