Hi Fortyfoxes,
As I have said previously, I am now totally convinced that this issue has
nothing to do with my ATEM Mini.
As a result of numerous experiments (including with external USB webcams directly) and hours of consultation with high level Apple Support people I have concluded that the issue has something to do with me using an external monitor.
I have a huge 43" HP monitor (HP Z43) connected with HDMI via a hub (thank you Apple!). I bought the monitor because it was supposed to work with USB-C but it doesn't!
What I've discovered is that if I disconnect the external monitor when the delay is occurring then after about 5-10 seconds the delay goes away and stays away. And if I plug it back in again it comes back! Furthermore the other day I was working in Final Cut Pro when everything started
really slowing down with a spinning wheel after every action. (On reflection this has quite often happened to me and I remember thinking that it really shouldn't be considering it was not a demanding task (4 camera HD edit) and I have a maxed out 2019 MacBook Pro!) Anyway, on this occasion it occurred to me to try disconnecting my external monitor and guess what? After about 5 seconds everything speeded up to what it should be. I then reconnected the monitor and the delay came back again!
When this problem first occurred the Blackmagic Design support person commented that the delay would be caused by the CPU being flat out doing other things thus preventing it from keeping up with the frame rates coming from the ATEM. At the time this made sense to me but I couldn't see what was using up the CPU. However now it seems that it has something to do with it trying to feed my external monitor.
I then thought that perhaps the problem is my particular HP monitor which was the only monitor of its size available when I purchased it last year that supposedly did work with USB-C. However, when I looked again last week I discovered that LG now have a USB-C 43" monitor. I figured, since Apple sells LG monitors, that surely there wouldn't be any issues about it working with my MacBook Pro so I purchased one. But, would you believe, it doesn't work with USB-C either and, furthermore, all of the above delay issues occur with it the same as they did with the HP!
We (Apple Support and me) are still working through the USB-C issue but, as the delay problem
only occurs when I have an external monitor plugged in I have no choice but to conclude that it has something to do with that and certainly not my ATEM Mini (which I have upgraded to the wonderful ATEM Mini Extreme Pro, by the way).
I'd be very interested to know whether the rest of you experiencing a similar problem also have an external monitor plugged in and, if so, whether you find disconnecting them makes the problem go away for you as well.
Wayne