There is a combination of factors. If UI3 works fine when you are local, then I have little reason to believe the trouble is caused by CPU usage, sub stream usage, or any other software optimization within
Blue Iris.
Your connection from China to Los Angeles is likely high latency with lots of jitter (variations in latency), which messes with Blue Iris's ability to send video reliably at a sufficient rate. The streaming pipeline between Blue Iris and UI3 includes only the minimum possible amount of buffering in order to keep delay at a minimum. This means at any moment when the network can't keep up, UI3 does not have a buffer of video to fall back on.
Do you get the orange clock in the upper right corner of UI3 after a few moments? If you right click (long press on touchscreens) the video in UI3 and open the "Stats for nerds" panel, you'll get some graphs which include among other things the "Network Delay" currently being experienced. If that graph goes up over time, it means your bandwidth is insufficient and you should choose a lower resolution streaming profile in UI3 or otherwise limit the streaming bit rate (UI3 has a global option "Maximum H.264 Kbps" in UI Settings > Video Player). That is one potential cause of getting a low frame rate.
And finally, if Zerotier can't make a peer-to-peer connection for any reason, it may try to tunnel through some other server which can further increase latency and reduce bandwidth available. I've had connectivity problems before with Zerotier that I could only resolve by manually forwarding the appropriate ports to the Zerotier service on important machines so that a proper peer-to-peer connection could be established more reliably.
