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I believe the only "delay" is with the system. I think it pauses for 1-2 sec regardless of hold time.
Here's my two cents;
In two words, "It's complicated". I think you have to carefully visualize how the scanner works in its quest to get you to exactly what you want to hear.
I suggest you dismiss the concept of "channels" as in the older model and give some thought to how the trunk tracking XT scanners operate. System->Site(s)->Group(s)
All of this assumes "ID Scan".
In scenario one, you'd have one System, systematically scanning through all Sites it can detect, then pausing momentarily to listen to the Control Channel to see whether any of the Groups you have enabled/programmed in that System are active - then moving on to the next Site/Control Channel, etc. after the delay time, if set.
This would seem to yield the quickest result in locating an active Group and more traffic.
In scenario two, if you have more than one System, with Sites and Groups divided between the Systems, the odds are greater that the scanner will be busy scanning the other System when your Group of interest lights up. Timing is going to depend on how much time the scanner spends listening to Control Channels in the other System.
I'll give you a real example based upon my own setup;
Being in CT, I'm sure you are familiar with CSP, the troops are split into three Districts; East/West/Central. The 11 or 12 troops generally light up on the same few Sites, based upon geography.
I have one System set up with all Sites and all Groups. This yields the most radio traffic because as it hits the Sites with an active CC it will look at all Groups/Channels I have enabled.
I have other Systems set up on individual SQK's for dedicated troop traffic, so each System contains the one or two CC's that I am confident through experience will receive the traffic for each of the 11 or 12 troops. If I turn on all of the individual Systems, it takes the scanner longer to go to each System, listen to the Control Channel, determine whether there is an enabled Group within the System - then move on back to square one. Therefore I am more likely to miss a transmission taking place in another System. The benefit is that I can select which troops I want to monitor more quickly and monitor only troop traffic and not all of the other CSP Groups. It's easier to follow breaking action.
This is not a scientific study, nor have I used a stop watch to prove my findings. It's only what I observe in my own scenarios. I hope it helps you visualize your own scenarios.