I was super enthusiast about Motion Live rig,
built a brand new Windows 10 pc, just (paid) Reallusion software installed.
Bought an iPhone XS for facial realtime mocap then connected everything successfully, and finally went live:
face freezes for half second every 3.5 seconds
That happens mathematically with this timing, I'm desperate.
I checked video posted by Reallusion and when they show facial mocap with iPhone, I just realized that every video cut lasts less then 3.5 seconds, so I don't see any example of that thing really working for longer time.... 

Correction: the freeze time is about 3.5 seconds and that happens every about 7 seconds.

I use it all the time for very long videos (minutes, in fact).

So, first things first -- how are you connecting, wi-fi or USB?  How is your wi-fi (assuming you're connecting that way)?  Router, bandwidth, what devices are also using it at the same time, etc.?  I assume your phone is up to date with the latest iOS and patches, but it never hurts to look (and to use the very latest Live Face).  You might want to post a complete systems spec for us to see and try and figure out the issues.

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Alienware Aurora R16, Win 11, i9-149000KF, 3.20GHz CPU, 64GB RAM, RTX 4090 (24GB), Samsung 870 Pro 8TB, Gen3 MVNe M-2 SSD, 4TBx2, 39" Alienware Widescreen Monitor
Mike "ex-genius" Kelley