You're right, I made an assumption! I'll test and check the responsiveness of JRiver to changes I make in a folder "deep inside the tree" and get back to you all.
Technically, the way MC's Auto-Import system works is using the
Filesystem Events exposed by Windows. This means that, in normal operation, it
does not need to rescan a full directory tree to pick up changes.
It is notified by Windows the instant one of its "watched folders" changes in any way, and it can act on only the changes. It does "full scans" generally only when:
1. It first launches (in order to reconcile changes that occurred while it was shut down and "not listening").
2. When manually commanded to do so via Tools > Import > Run Auto-Import Now
There are some (intentional) delays built into the processing in order to allow the filesystem to fully commit changes before MC acts on them. And, actually because the filesystem change events come in before files are even fully written (while a big file is copying from one volume to another, for example), it has to check repeatedly and then sleep.
But it does not generally just "rescan" the "watched paths" on a recurring schedule. It is way smarter than that.