Constant buffering, logs showing very low speed

Plex Media Server
6

Glad you have it working.

Deleting the file is not required. You can edit Preferences.xml instead.

Note that hardware transcoding of HEVC HDR video at Level 5.0 or higher will fail. The system will fall back to software transcoding.

See Synology FAQ 20: Bypassing the iHD video driver.