After upgrading my iPad Pro to iOS 10, I noticed what seems to be buffering happening on content that used to Direct Play without any issue. I checked CPU usage on the server and it was low (<50%). This content gets played daily (exercise videos), which is why I was sensitive to it.
I consulted the recent topic here: https://forums.plex.tv/discussion/232631/known-playback-issue-on-ios-10#latest
Disabling Direct Play seemed to help until the next day. I then adjusted the DNS of my ISP gateway to Google, no change. I changed the "Transcoder default throttle buffer" from default to 360 as it seemed like the buffer was not large enough, again -- causing buffering, no change.
I've attached the Plex Debug Info from my device. Let me know if there's anything else needed.
I do understand that iOS 10 is new and there are reported iOS 10 issues, but the issues stated other places looked like playback was not possible at all. I was ~30 seconds into playback and it would buffer for a bit... then play for ~1 minute and buffer.
My Apple TV seems to be OK on iOS 9.2.2 and I just watched a ~42 minute show with no interruptions. I did turn off Direct Play when this started happening as a precaution though.