• No data items - routes, tracks or waypoints - are imported automatically after a reboot. They have to be imported manually.
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• You can select which gpx file(s) from which to select data, also whether you want to import tracks / routes and/or waypoints. This restricts the list of items from which you make the final selection.
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• Only the selected items will be imported to Active Memory.
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• The XT2 will allow you to import the same route or track twice. Subsequent imports will have a number appended.
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• You cannot import Waypoints more than once. You can try but it doesn't do anything.
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• Track data is not deleted from the GPX file after it has been imported.
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• Any Item can be selected and deleted from the XT2 and re-imported from the original gpx file. So on a tour, if you want, you can clear your Active Memory of Routes Tracks and Waypoints, and import just the ones that you want for a particular day. (That is not necessary - since using collections will allow you to see on the map just the items in one collection.)
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• The XT1 is unable to find waypoints on the SD Card if it doesn't have one already in memory.
  The XT2 does not have this problem.

You may notice in older posts about other Zumos that I have recommended using Internal Storage for GPX files. Not any more.
For the XT2 I always transfer my GPX files to the SD card. The files cannot get altered and nothing gets deleted.
More than anything - the behaviour is predictable. And if your Zumo Collections get messed up, you can delete the lot and import them again.

It is traditional to place GPX files in the Memory Card\Garmin\GPX folder - You have to create the GPX folder yourself.

But the XT2 will also read GPX files from the Memory Card\Garmin folder.
Also from the Garmin\GPX folder - if you care to create the GPX folder there.

Subfolders with any name can also be used. eg Memory Card/2025 Routes/Spain. It seems to just traverse the directory folder tree and locate any gpx files no matter where they are. The XT2 reports the full path of the folder name, but it has only so many characters before they fall off the edge of the screen.

I would imagine that many files stored in a complex directory tree would result in an extended time in processing/importing. But in tests, it seems to be plenty fast enough.