I always wanted to talk to you about that subject,
@Steve. According to my experience this setting depends on camera+lens combination. Let's say we have set to some number like 3. That means that camera waits so to say 3 units (let's say milliseconds) and then tries to re-acquire the focus. If I have a professional lens (10k+) mounted then this combo will get the focus immediatelly or very fast. If I have Sigma contemporary mounted then it takes the next 3-4 milliseconds (or seconds ;-) ) until it will focus! because motor is so slow. I tested it with 500PF (fast!), Nikon 200-500 (quite slow in comparison to PF), Sigma (slow), Nikon 400/2.8 (fast). For Sigma I always have fastest speed in this setting.
I use Auto-Area AF on D850 and my keepers are much higher then with other settings (earlier I used 3D, groups and points). And I think this is becasue the algorithm is very easy. I mean it is easy to develope in software. The methode for DSLR is "take the closest object". The processor works fast with it becasue it doesn't need to make other sophisticated checks.
For z9 it is different because it has contrast-AF but behind it there is still a piece of software which can be optimised, a processor which must work with it, the interfaces which will transfer the information (communicate with lens) and a lens motor which will move the glass-elements to focus. All the parts mus work well together.. There is actually no single point of failure. There are many points.