listing all products(Manage Products) under admin panel is very slow

So when I click manage products under admin, It will load for me really slow. I debugged briefly and I saw that before Products get loaded, other things like Categories, Manufactorers etc loaded as well.
Basically I have many categories and it takes too slow for me to load categories.
In the ProductController.cs line 773 (version 3.5)

line below is executed really slow. What is it for and Can i somehow optimize it?
why are the categories loaded anyway? are they not cached? it should be fast to load from the cache

   //categories
            model.AvailableCategories.Add(new SelectListItem { Text = _localizationService.GetResource("Admin.Common.All"), Value = "0" });
            var categories = _categoryService.GetAllCategories(showHidden: true);
            foreach (var c in categories)
                model.AvailableCategories.Add(new SelectListItem { Text = c.GetFormattedBreadCrumb(categories), Value = c.Id.ToString() });

Caching of categories are not supported on the product list page. How many categories do you have?

a.m. wrote:

unfortunaly we have around 75 thousand categories with the all childs and parents.

That's a lot. In such cases some caching in presentation layer should be supported (similar to Nop.Web project). Please find this work item here

The feature is implemented. please see commit here.

Thanks a lot for suggestion!

UPDATE: we've just added caching support for manufacturers and vendors (admin area)