I've been using my two Roku Ultras with Plex Server running on my computer's hard drive without a problem for about a year. One Ultra is Ethernet-connected, and the other uses wi-fi in a small house with Comcast Xfinity service. I recently bought and configured a Synology DS920+ NAS unit, installed the Plex Server package to it, and moved my media onto it.
Now, if I use the Roku Plex app to view videos using Plex on the NAS, the experience is less than optimal. In particular, my home videos - taken on my phone at 1080p - take a long time to buffer, and then they'll stop and buffer some more before I get to the end. I never had this problem before the NAS.
My TVs are new - both smart LGs, one a 65CX OLED, the other a 55NANO LED. On both, I installed the Plex app and I can watch all of my videos without the buffering issues that I see using the Roku.
I'm imagining there are settings that I'm not familiar with that can make my Roku experience again acceptable when viewing from my Plex Server. I would appreciate any help or direction to a source of information.