The video is HEVC. Other than Safari, web browsers do not support HEVC video, so Plex must transcode the video.
You do not have a Plex Pass, so all transcoding is performed by the CPU. The Celeron CPU will struggle with 1080p material, especially HEVC, as it requires more resources to transcode than H.264.
You also have subtitles enabled. This puts an additional load on the CPU, as it has to burn them into the video stream. Note that this process always occurs on the CPU, even if hardware acceleration is enabled.
Option 1: Use a Plex client that direct plays HEVC video.
If you are using a Windows or Mac system, use the Plex for Windows/Mac application instead of a web browser. Plex for Win/Mac direct plays most content, including HEVC video. Note that this essentially transfers the transcoding from the server to the client. You may see the client CPU utilization increase. You may still experience buffering/stalling if the client has a underpowered CPU.
Option 2: Use Hardware Accelerated Transcoding (Plex Pass required)
You can also purchase a Plex Pass and enable hardware accelerated transcoding. This will shift the transcoding process to the Intel HD Graphic 500 embedded GPU.
Note that subtitles will always be a problem when using Celeron based NAS. As mentioned above, subtitles are added to a video stream by the CPU, not the GPU (an Intel driver limitation). The process is also single threaded. The J3455 has a low single thread rating, so it will struggle when burning subtitles into the video stream.
The end result is that the DS218+ can easily transcode 1080p video using hardware acceleration, but you may still see buffering when subtitles are enabled and the video is transcoding.
You can try a Plex Pass via the monthly subscription ($4.99 USD). If it does not help you can cancel the subscription. If you wish to keep the Plex Pass you can consider the other purchase options if desired (yearly, lifetime, etc).
References:
Plex for Windows/Mac
Plex Media Server Documentation
- Using Hardware Accelerated Streaming
- What kind of CPU do I need for my server? - See Passmark ratings for software transcoding
- Plex NAS Compatibility List - See spreadsheet listing NAS transcode capability.
Intel J3455
Plex Media Server on Synology
