Opening the WorkShop is almost like opening a game within the game, or starting the CK within the game because the WorkShop needs access to the deeper layers of the game, contrary to when 'just' running around shooting-up the neighbourhood. The reason why the WorkShop lags a bit is obvious as everything in the game needs to be made available -especially when using scrap mods- not only the textures, but all assets involving all items that can be scrapped or added. The last ones I added ( Art Connoisseur System Frame Work and the Atom Bomb Pin-ups that uses the ACS) makes my game freeze for at least ten seconds when hitting 'V' or otherwise opening the WorkShop.

Eight of those mods have a Script Injected menu, which means they have their own category in the WorkShop. Now with my most recent game, I'm at 291 mods with 198 plug-ins. This is because Fallout New Vegas is an extremely fragile. A compact, gameplay friendly mod list with a solid core and engine fixes is a perfect way for you to start modding this game You may notice that this mod list is smaller than usual.

It used to be almost instantaneous unmodded and like you around two to three seconds with some serious modding (339 mods of which 254 had a plug-in). ESO and Ultimate Immersion presenting the new Fallout New Vegas mod list. To further answer TwilightOne's question(s), loading times for the WorkShop for me are now between ten and twenty seconds.

Fallout 4 textures take forever to load