WordPress 3.6 introduces new accessibility functionality in the Appearance > Menus admin screen. This extra javascript processing creates inefficiencies when handling menus with large numbers of menu items, resulting in very slow and sometimes unresponsive interactions when managing menus in WordPress. By deferring this processing using "lazy" techniques, we can eliminate the bottleneck and have the menu management system run much faster for large numbers of menu items. This plugin does just that.
More information: Speeding up the Appearance > Menus Screen in WordPress 3.6
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