It sounds more like a slow connection issue…BUT there has been some issues with hardware transcoding with the J4125
See this thread
Two things to check. Verify that your server is properly configured for remote streaming. If you have a relay or indirect connection you are limited to 720p 2mbps stream with a Plex pass
On each client there is a quality setting you can also adjust. 2-4mbps is usually the default for remote streams. You need to switch that to Original or maximum
Your NAS is not the problem, unless it’s related to the bug in the thread I linked
