New accessibility JavaScript present in WordPress 3.6 Dashboard -> Appearance > Menus panel insignificantly slows down the loading speed specially when you are having a large number of menu items (> 50) to manage.
Faster Appearance – Menus is the new WordPress plugin created to solve this problem. Simply install and activate it. After activation the plugin defers the accessibility processing to eliminate scripting bottleneck and allows Menus admin screen to run faster and more responsively.