Why is Capture One taking so long to open? What is it doing?
By default, Capture One will open any documents that were open at the moment you closed the application. Consider that Capture One can open two document types: Catalogs (cocatalogdb) and Sessions (cosessiondb). Those documents render previews, process files, etc.
Think of them as air traffic control for Capture One.
Keep the document on a fast internal drive
These documents are constantly being read/written to, and in the case of catalogs can become quite large. Under the hood, they're SQLite databases. The document is optimized to be handled locally; that is, not over a network. To have the best performance in Capture One, it's advisable to keep these sessions and catalogs on internal drives with fast read/write speeds. Additionally, the SQLite database format is meant to be client-side, not server-side, further optimizing the local document storage.
You can read all about SQLite here: https://www.sqlite.org/index.html
To continue the air traffic control metaphor, think of this as keeping the control tower within the site and radio transmission reach of the planes.
View a small collection prior to closing
As mentioned, the last document will be opened and that document will be returned to the state that it was last closed in. This might be viewing a collection of thousands of images, which will mean that the previews need to be pre-loaded. Undoubtedly, loading a few hundred previews is going to be faster than a few thousand. Prior to closing Capture One, choose a collection that has few images in it. The next time the document is opened, it will only have to cache the previews of that collection.
Managing a small regional airport, while no small feat, is much less cumbersome than a hectic international hub.
Use the correct Cache Preview size
Setting a cache preview correctly, as noted in this article, will reduce the rendering time needed when opening a document again. If the cache preview generated is the correct size, no new preview will need to be generated and everything will be smoother, from loading to rendering adjustments to the previews.
Your runways should be an appropriate size for the jets landing at the airport.
Keep up maintenance on the database
Particularly with catalogs, it's important to perform regular backups and include the Optimize and Test Integrity options. This will keep the database error-free as well as prevent corruption. You can also test and repair a database via the steps outlined in this article.
The tarmac should be as smooth and pothole-free as possible.