If you as admin create a category and set it to be a members' folder, the category will display like
this where any member who uploads to that category gets an automatically created user name sub-folder. You can also allow members to create unmoderated sub-categories in a category you create. If you don't want to allow unmoderated sub-categories, you can allow members to suggest categories instead. As for server resources, there were a couple threads about slow queries, but this was addressed with the pro v.2.1.4.5 release, and there haven't been any more threads about it since that time to now. Also, each page of the online
demo has text in the footer: page generated in X seconds with Y queries. Note that the footer text may not be exact, because for example your site and this site may have different server setups, modifications, etcetera, but you can use that text to get a sense about server resources, at least with respect to this site.