Hi I have, qnap 469l 4x4tb hd with max ram (located in large cupboard, new one to go same location) Cable cat5e to devices 8 way 1g switcher AmpliFi mesh system Nus i7 intel pc attached to 65lg TV plex and kodi 2 x32 lg TV plex Wired Bose sound touches. Plex buffers a lot, kodi slow loading What to update NAS with existing drives, add 2 external cameras and 2 indoor. Main use media and general photography Buget for NAS £700
If you have connected NUC PC via HDMI cable and it is buffering and slow loading that is strange. But if the other TV buffers accessing over the Wifi/Ethernet then you are probably trying to transcode some heavy 4K files. If you have no 4K TV you could get a 4K HDMI dongle such as Android TV or ShieldTV, this would allow streaming original videos without converting them on the fly.
Nas in this price range would not do any better job compared to i7 NUC.
TS-453D could stream 4K via HDMI or over the network to a compatible TV. Transcoding of 4K could be only achieved using the Qnap video app, not Plex.
The new 73A series from Qnap allow PCIe graphics cards. I am looking forward to running 4K tests on those.
I hope this helps.