I'm now playing a bit with your ffmpeg exec flow with multiple streams. Main problem is my Lorex DVR sub-streams are CIF (352x240) only a bit bigger than your Big Buck Bunny public rtsp stream. the HD streams "choke" the Image Output node resulting in displaying mostly garbage.
I took a cell phone video of my i7 Desktop running 9 HD rtsp streams into my Python AI and my node-red "controller" dashboard that displays a single selected channel for testing and/or camera positioning.
Quality is not great but watching the time stamped file names update in the node-red UI and the pool cleaner moving in third 3rd window from the top, adjacent to the browser window, gives a good idea of how it runs. The AI was processing an aggregate of ~28.3 fps using 1 Movidius NCS thread + 1 CPU AI Thread, + 9 threads to process each rtsp stream, and the main thread to draw the displays and send the MQTT messages to Node-Red.
Python AI and Node-red Dashboard Controller in action.
I need to get one of those "tripod adapters" for my phone -- results, while quick and dirty but useful, would be a lot better without the camera movement and resulting focus jitter.
I opened a "ticket" with Lorex tech support asking why the D1 sub-stream settings revert to CID shortly after applying. They've never bothered to reply before to any of my questions so I don't expect any solution.
D1 video is perfectly fine for the AI, actually I expect CIF would work too since the AI is 300x300 pixels, but a CIF image is inadequate for viewing in Email to make a decision about the AI notification (UPS guy or Criminal at my door) -- that's where the 720p and 1080p really have the advantage.
I'll try to setup 8 of my CIF streams and your BBB stream and post another short video clip.
This morning I setup a fan-less 12V powered dual cor i7 "NUC-like" computer, the AI code on it with 9 HD rtsp threads and the same 1 NCS + 1 CPU thread is giving ~18.4 fps. I've also got an i5 version of it I plan to try soon. The i3 version of it gave ~12.6 fps with 6 HD rtsp streams -- my feeling is fps/Ncameras ~ 2 is about ideal give or take a few cameras 
Next priority is moving the code to OpenVINO so I can compare NCS and NCS2 sticks.
