The vim3l is connected via ethernet port directly to my switch (ubiquiti 48 Pro poe gen 2). The plex server is on a separate physical machine (i7 7700k) and connected at 10 gigabit speeds to the same switch (sfp+). All files are hosted on a separate physical freenas server connected at 10 gigabit by sfp+. Iperf shows 1.2GB/s between plex server and freenas as expected with smb at 600MB/s.
The file is a hevc and I have enabled both of the options that you mention.
Edit: the buffering occurs when using kodi, not plex (plex doesn’t work at all). Freenas is mounted via smb on the movies section. Is it possible to speed test the ethernet connection?