Please note that as of WordPress 4.2, the patch that this plugin provides is now incorporated into WordPress core, so the plugin should no longer be necessary.
If you have a lot of menu items, you’re probably experiencing a lot of lag in your Appearance > Menus panel. This is due to some new javascript processing that WordPress Core does to add accessibility features to menu items.
In my tests, when you have about 100 menu items you’ll get about 5 second lag. That’s pretty annoying.
So I’ve written a plugin you can install that will greatly alleviate the javascript bottleneck in the core. Keep in mind it is a beta plugin, and it’s possible there are some bugs. But if the slowness of Apperance > Menus is bothering you then I’d definitely encourage you to try it out!
Here’s an article on the technical assessment and proposed resolution of the issue:
Speeding up the Appearance > Menus Screen in WordPress
The plugin is available in the WordPress repository and has helped many users speed up their menus panel (with or without UberMenu).
Note that if you have a huge number of menu items, you’re still going to experience some amount of lag no matter what. One good strategy to avoid this is to break your menu down into more manageable chunks via Menu Segments.