So,
Just installed v9.0 on my Windows 10 box. I’m using internalsql with the mariadb executables.
Digikam v 9.0 opens fines, all my photo info is there. Great.
About 5 minutes later, it blinks and I’m presented with the Welcome screen.
My database is stored locally d:/digikam and my images are on a shared folder on my Synology NAS. W:/
What’s going on here?
If I reset the location of the image warehouse, it will then rescan the folders.
Why isn’t it using the existing database?
Never mind:
Dang, I just now downloaded and installed the new version on a different computer. I need to go check on it now.
Well,
V9.1.0 is finding the database, but “Find new items” is stuck at 0%.
Perhaps I’ll regress to v8.8 
LOL.
Well, it did finish the scan. Just no indication that it was actually working…
Wow.
It found some new “faces”, so I went through and accepted, deleted, ignored, creating about 500 images to syncronize.
Hitting the synchronize button about 20 minutes ago, I’m at 40%.
Wow. This is excruciating. I NEVER saw this sort of thing with v8.8.0.
Regression is looking more and more likely at this stage…
Well,
The secret is to turn off “Monitor albums for external changes”.
That turned it from nearly unusable into fairly responsive.
Geeze, what a PITA
Ok,
Some feedback for potential upgraders, particularly if you are using the Windows version.
Make a copy of your database and config files before starting.
Seriously.
Let me repeat:
Make a copy of your database and config files before starting.
Once you have migrated to v9.1.0, you can’t go back to v8.8.0.
Something is done to the database in the conversion to v9.1.0 that makes the data incompatible with the previous versions, even if you are using the internal mysql tools. Dunno about external.
If you are on Windows and your images are on a NAS, make certain you install v9.1.0. V9.0.0 won’t work. Get it from here: KDE - Experience Freedom!
And, like I said above, turn off “Monitor albums for external changes”. Having this on heavily beats on Samba. I had something like 4,000 processes going when I checked.
Even with all this, I’ve finding response very lethargic. Not unusable, but not great.
I’m in the process of cleaning up my tags and such in metadata, but when I’m done, I’m thinking about writing all the data to the files, and starting from scratch, perhaps going back to v8.8.0.
Good luck.