Hi Annet,
Would you mind to check how large the Folder with all assets is, and the scene file as well?
To get an idea about the time it takes at least, save an empty scene file.
Since I have to guess here the reason of the slow process, my least favorite way to do, if all the images are stored while you saved it, then there was (perhaps) a larger data transfer needed. If so: You can avoid this, by either determine the place where the image are (Preferences>Files>Asset Paths). Which means the images are not copied, they are just “linked”, if “Save [as]” was used. Or, to manually create a project folder, and inside of that folder create a folder called “tex”. Inside of the tex folder place all the images you want to use. Pick the images from there while creating your project. Save the project file from the start in the project folder.
If you have all on one hard drive, and your book has maybe 100 tiff images – each 100MB large, while the drive has USB2 speed only, then 10 minutes would be even fast, as the images have to be read, and then written. Again, guessing is certainly no my understanding of professional advice.
All the best
More detailed information about,
Preferences>Files:
https://help.maxon.net/us/#PREFSFILES-PREF_FILE_MAIN_GROUP
or
General Project Save options:
https://help.maxon.net/us/#5595