sevenspark

The new accessibility javascript in the Appearance > Menus panel in WordPress 3.6 can significantly slow the page load speed when using a large number of menu items (> 50). This plugin defers the accessibility processing to eliminate the scripting bottleneck and allow the Appearance > Menus screen to run much faster and more responsively.
<a href="http://sevenspark.com/wordpress/speeding-up-the-appearance-menus-screen-in-wordpress-3-6" rel="nofollow ugc">Full details</a>
This solution is fairly rough. Hopefully we can develop a robust solution that can eventually become part of the core. Pull requests through GitHub are welcome.

Faster Appearance – Menus vs ElasticPress