Well, it sounds first like something might be competing with Endnote for resources, because it never takes me long even in a thesis with many citations, figures, tables, etc.
But if you are willing to forgo the bells and whistles, do what I do. Turn OFF instant formatting (on the endnote ribbon) and add your cited pages manually.
basically – add the @ and pages and they will be formatted (assuming you have the cited pages field included correctly in the citation template) when you update the citations and bibliography.
{Gross, 2009 #26@51-52}
you still add the citation from endnote, but modify without having to “edit citation” rigmarole.