The dynamic menus are basically drop down menus which appear when you move your mouse over the top menu items (the ones on the navigation bar). They will list all the items in each section to allow for quicker and easier navigation.
A number of people had problems with page loading speeds so it was made a user pref. You can try them and if they slow you up, simply disable them.
I am in the process or rebuilding the system to be less flexible but much faster.