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.
In my tests, when you have about 100 menu items you’ll get about 5 second lag. That’s pretty annoying.
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.