Yes, there is always a performance hit to produce WYSIWYG menus in any application. We have done what we can to minimize any performance issues by caching this information. The biggest performance issue you will see is after QuarkXPress and System Font Caches are deleted. We also initialize the font menu during launch so there may be slower performance when launching QuarkXPress, based on how many fonts are loaded. There will also be a small hit the first time the Font menu is viewed in a QuarkXPress session. After that it will become snappier. There may be some delay on a family submenu if the newly activated family has a lot of faces (for example, the 168 faces of the Kepler font).
If you are dissatisfied with QuarkXPress 8 performance in this area, you can turn off WYSIWYG Font Menus by going to QuarkXPress > Preferences (Mac) or Edit > Preferences (Windows) and unchecking Show in Font Menus in the Fonts pane. You can also temporarily disable WYSIWYG display in font menus by holding down the Shift key before displaying the font menu.