I'll take a guess ... you need to optimize the catalog. File->Optimize Catalog.
I did. Optimizing made no difference at all.
- Tony Cortizas Jr.
Sent from my iPhone
Do you have a backup of the catalog in question? If so, How long does it take to open it?
If the same length of time then the backup suffers from the same ailment.
Create a new catalog. Does that open in a reasonable amount of time?
Import from another catalog (the original 8min) into this new catalog.
In any case I will assume a reboot of the machine with no other applications running.
Dear Adobe Support - the mystery continues.
Thanks, commenters, for your suggestions. After all else made a difference, I did indeed start over by making a new catalog.
Basically, I can make a new catalog (e.g. 16K images) and it takes 3: 40 minutes to open. Then I "remove" 3K worth of images from the catalog and the catalog opening time does not change. The "previews" file size is indeed smaller, so I don't understand what really determines the opening speed of a catalog.
Optimizing the catalog -- multiple times -- makes no difference!