This is a known issue in some environments. While a simple "SELECT * From [dbo].[Dependencies]" will execute quickly in SQL Manager, the page will not load quickly because of the JOINs necessary to get all the data for the page to display. A dependency parent may be a Node, Group, or Interface, and a child may be a Node or a Group. Hence, we join these tables to the Dependencies table in order to get the Name and Status of these dependency relationships.
Performance will be slower when you have more Groups, and in particular more Groups that have Dynamic Queries associated with them.
Your environment sounds too slow for usability though, so I suggest opening a support ticket so that support and development can take a first hand look and make improvements to this page in a future release.