Hello
We open a store using nopcommerce, the store need to hold 10 of  thousand of products, and there are more than 7000 categories!!!
only 20 main categories, but there are many sub categories
the drop down function in the manage product is just not build for this kind of mission and loading the drop down in 8 minutes!!!
there can be other solution for this, like ajax tree view, auto complete categories and more, ajax dropdown or other staff.
thank you.

Resource
Presentation\Nop.Web\Administration\Controllers\ProductController.cs
Function
ActionResult List()
Model - ProductListModel