I would try using the latest beta Client version located here: Release Announcements - Android TV beta
You are also on a relatively old Nvidia PMS Server version. Get access to the latest 1.15.x beta builds via this method:
https://support.plex.tv/articles/201844613-early-access-beta-releases/
I would also make sure you are on the latest Nvidia Shield TV 7.2.2 OS release.
There is also a known issue related to bluetooth & usb 3 performance. See here for more information: https://www.reddit.com/r/ShieldAndroidTV/comments/a70xl7/usb_3_and_bluetooth_interference_questions_is/
That isn’t an Nvidia specific issue but affects many devices.
Also using an external USB 3.x SSD is going to be faster than USB 3.x HDD (especially when multiple things are happening at once) due to limitations of single HDD capabilities, etc.
