Don’t panic.

First, it backs up all the data that you (User) had a hand in creating. It does not backup the Factory Presets (this is unnecessary). It does not backup the current Firmware (this would be impossible).

What you want to do is check on the data that will potentially be in that Backup File. Here’s how:
Press [UTILITY]
Touch “Contents” > “Data Utility”
Here are the Folders that contain the data that you have created. Touch each Folder, in turn, to view its contents.
To return from within the Folder, tap the box “>MONTAGE” at the top... it shows the path when navigating the Folders.

Arp - If you created up to 256 User Arps, they would appear here... otherwise the folder will appear empty
Library - your eight Libraries will appear here... includes their Arps, Live Sets, MSeqs, Performances and Waveforms
Live Set - your eight User Banks (each with 16 Pages/16 Slots)
Motion Seq - If you created up to 256 Motion Seq they would appear here...
Performance - If you created up to 640 they will appear here
Song - up to 128 Songs
Waveform - up to 2048 Waveforms currently in the USER Bank... the Waveforms will be backed up whether or not they have been used in a Performance program.

Only the Library and Waveform Folders would contain massive amounts of data (in the MB range).

But it is the data contained in these 7 Folders that make up your Backup File... plus the current Utility Settings.
If all of these folders are empty, your Backup will indeed, be very quick.

Ah! That explains it. Thank you! My assumption that EVERYTHING was copied was way wrong, as I should have known. My User data consists only of Pfs and associated waveforms actually in progress. No Arps, no Motion Sequences, no Songs, no Live Sets (a pretty bleak soundscape!) so it wouldn't take long to back them up. What's on the USB would be ignored as well (I keep a copy of that on another USB). That's put what passes for my mind at rest, thanks BM ...