It occurs to me that the advice has always been to use a single catalog rather than many. Having done that might have minimised the impact; the update on a single large catalog might take longer but would probably take less time than the update of many smaller catalogs.
Apart from what Victoria and others have mentioned, I would like to add a few more details about the catalog upgrade.
An upgrade may be required to change the database structure according to new features or may be some other optimisation.
With the 7.0 release, we have done some changes to the internal representation of the data(develop history and image metadata) which helps in reducing the size of the catalog(we have observed approx 50% size reduction in our internal tests and similar results have been shared by other customers) and helping the performance as well as far as read/write to the catalog file is concerned.
Catalog upgrade is disk intensive operation, so it may take some time to complete. In our internal tests upgrade for a 6 GB catalog took about 25 minutes on SSD and 40 minutes on HDD.
One of the reasons for the upgrade taking longer that expected could be lack of space on the corresponding disk drive/partition. Catalog upgrade may need free disk space about 2x size of catalog.
Can you check if that is the case? If clearing disk space does not help, let us know, we can debug the issue offline.