Is Synology DS218+ a good NAS for Plex 4K playback?
Perfect for 4K streaming
It's important to get the differences between decoding and transcoding out the way first. The former is the decoding of media for playback. The latter is far more intensive since it requires the host device (in this case the Synology DS218+) to not only decode the media file, but also convert it to another format and then stream it if the destination doesn't support the original file format.
So say you have a 4K TV and the DS218+ hooked up on the same network. Both support the same 4K file format (.mkv in this example), you're good to go. The DS218+ will stream the 4K content to the TV and you'll be able to enjoy your favorite movies stored on the NAS. If your TV doesn't support the same file format (say it's an older TV model), you're going to have to downgrade it to 1080p.
Not powerful enough to transcode in 4K
The Intel Celeron J3355 2.0Ghz processor found within the DS218+ simply isn't capable of handling 4K transcoding. Not many NAS models from the majority of major vendors include processors with the right features to facilitate 4K transcoding — it's just something we're going to have to put up with.
The DS218+ can transcode up to 1080p, so not all is lost. Just make sure your output devices can handle the same file formats you plan on streaming and the DS218+ will be a perfect Plex server.
Only with Plex Pass
One thing to note is the fact hardware transcoding support is only included with the Plex Pass. If you're rocking a free account, you'll need to rely on software transcoding, which makes things even more restricted. You may encounter some buffering or other related issues if relying on this method.
Paying out for Plex Pass will ensure you're taking advantage of the hardware you have available. Pricing isn't too expensive either. You have the choice of three plans: $5 per month, $40 annually, or $150 for lifetime access. One-off payment of $150 would save you a lot in the long run, especially if you have everything set up as you like.
Powerful enough for most NAS tasks.
Like most other NAS units available for purchase, the DS218+ from Synology wasn't designed to handle the transcoding of media in 4K. You can stream 4K content to other devices that support the file format without much issue, however.
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Rich Edmonds was formerly a Senior Editor of PC hardware at Windows Central, covering everything related to PC components and NAS. He's been involved in technology for more than a decade and knows a thing or two about the magic inside a PC chassis. You can follow him on Twitter at @RichEdmonds.