Giles2012 wrote: … The photos are on an external hard drive.
How is your external drive formatted? To avoid damaging the Photos Library an external drive must be formatted in either APFS format or Mac OS Extended (Journaled) format. There have been so many problems with using incompatible drives that the newest macOSs won't even allow a Library on a non-Mac formatted drive to open, since there is a chance of damaging the Photos database. See this:
Move your Photos library to save space on your Mac - Apple Support
If this drive is in a an incompatible format, stop running Photos with it immediately! A Photos Library can sit on an incompatible drive, but running it may corrupt the database.
Photos uses a database to keep track of the pictures, your edits, and all the data that goes with each. With a new OS, the amount and kind of data changes, so the database has to be updated, as well. Since you can have several Libraries on a Mac, this can't be automatic, and it is done when you first open a Library. It takes maybe a couple of minutes. If yours is taking much longer, then something is wrong. My first guess is the format of the drive.
What did you find?