Cheapest NAS for Plex that can steam 4k h.264 without buffering every 5 seconds

[Enquiries]

I'm sorry I'm so challenged here. I have a DS420+, it can stream 4k plex h.264, but there's a lag every 5 seconds. What is the cheapest NAS that won't have this issue?

[ed]

Sometimes if this is a local streaming, it is worth converting a video into a compatible video file that TV can play. Or sometimes it is better to buy a new TV that supports all video formats.
If this is not the case and Plex itself causes issues. You may disable transcoding option. Or disable subtitles that may cause transcoding.
Go to system monitor and check if CPU or RAM running high. This will indicate if transcoding is activated. It should not be in local streaming.

I hope this helps.