Thank you once again Glynor, this made a marked difference in performance in my case.
(my offline drives are USB not network)
My offline disks are SATA (using one of those SATA drive docks). You don't even need to disable that option as long as you isolate the files that might be missing from your regular library views.
The issue is that when you open a view, if that option is enabled, it checks the status (available/missing) of
every file that shows up in the view (even those not onscreen, scrolled down at the bottom). If you open an "all files" view that has several thousand assets visible, but missing, which are mixed in with those that
are available, the view will load VERY slowly with that option enabled. It has to check
each one. This is even worse if the files are on a network drive for the reasons Hendrik explained. The problem is that it is updating the UI thread, I suspect, because of the little X pictures, and the UI thread is always blocking on Windows.
I solve this by having special "All Video (including Offline)" views set up in my Library. Stuff on external (only sometimes connected) disks are isolated to those views. I still get a delay when opening one of those particular views, but my regular views that have no offline files (the online primary media volume) open as snappily as you'd expect.
I can post additional instructions on how I have this set up if you're interested. I basically use a combination of a user-defined field [Archive Drive] that stores the "name" of the offline drive (making it easier to look up which drive contains which files), and a calculated field [Offline] that determines if the file is on one of those drives.
Then I exclude all files where [Offline] = 1 from all of my top-level Audio, Images, and Video views.