It would seem odd for it to be a problem since the alternative board-side would be to make demands on the board at least twice to get where you are going rather than a single time using the forum-jump-dropdown-menu. Perhaps, to go from one forum to another, not having forum-jump-dropdown-menu is actually contributing to the slowdown as thousands of users are having to make twice the demands on the database as they need to click from the forum they are in to the forum menu then click on again to the forum they wish to actually visit. Does going to the forum menu not put demands on the database?
No - the menu has to be dynamically generated with every single page load on the whole site. It's a pig.
A static version like on CM might be the answer, as someone suggested above.