Happened to be browsing the forums and came across this thread: Nvidia Shield TV pro 2019 - very slow Plex app since 8.2.1 firmware
Disabling opportunistic lock on my synology solved this issue for me. Videos play almost instantly via the native Plex app for shield (used to take around 20-30 seconds to load any file) and now all media finally plays within the Plex addon app for Kodi.
