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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.
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
directory to the /wp-content/plugins/
directory, or install via the Plugins uploader