I could not figure out what network activity was happening on my network or why spike in disk activity was happening on my NAS.
I finally dug down into it. Found it was plex, then turned on verbose logging and found it was a family members ipad syncing content from my server. It was not showing up in the sync status of the server home.
It's a pretty annoying bug on top of the massive framerate slowdown when controls are visible on the newest Web UI. I wish there were a proper place to submit bugs.
(Restarting Plex did not fix it)