Live video in sync - browser video out of sync.

Hi, I have recently noticed that sometimes one or more of my cameras lags the time slider time in the browser (and the other cameras.) The live feed for the camera shows the clock overlay ticking in perfect sync for all of the cameras. Sometimes it's one camera, sometimes several - I can't discern a pattern.

The video itself is delayed - it's not a clock drift/overlay problem.

I am not recompressing, and the total system load of security spy is about 30%. This is a 2018 Mac mini.

This is all with continuous capture videos.

3 of the cameras are Unify G3 Flex, and the other 5 are older Grandstream GXV 3610 FHDs.

What could be causing this?