How to restrict the assets of a particular folder from appearing in global or filter search results based on role of a user

How can I nest my Sub-Brand tags in my Brand tags and restrict the options in dialogue box for sub-brand tag based on the tags nested under that particular brand tag

Performance of DAM is overall slow so we upgraded it to 6.5 latest version and still we are facing the issue of slowness. Please suggest some ways to improve the performance and we are also following all the best practices suggested by Adobe.