Catalog 1: 277,198 (3.5GB) SLOW ..... As others mention. Switching between L and D is just painful.
Catalog 2: 135,089 ...Smooth, yet still a pain to having the need to switch from L to D, then adjusting second screen to the different view due to it automatically switching from changing modes.
Catalog 3: 9500 ......Snappy, yet still a pain to having the need to switch from L to D, then adjusting second screen to the different view due to it automatically switching from changing modes.
Made up mostly of roughly 11mp, 14mp, 22mp, some 44mp files Then there are the large PSD's and TIF's that can be as large as 2-3GB (mostly on the small catalog).
running on Dual SSD, 16GB ram on 3.4Ghz quad i7. My previews are 1440 med quality.
If you are organizing for something like a "stock" images, it makes sense to have them together as mentioned for the sake of cross searching.
If you have a specific type of work you do for clients, like Automobiles, or Food....I can see how a dedicated catalog would work for all your automotive (if it is large) clients or just "Product" vs Nature, Street, Arch, Family, Trips...etc.