GuyPrimavera

This open source and lightweight plugin allows users with the role Editor or Shop Manager to access the Menus and Widgets areas of the Appearance menu in WordPress’ admin area.
This is a common task that would be useful for clients to manage themselves to give them more control over their website’s content.
This plugin also gives access to the Customize submenu and other theme options, but these can be hidden through the options page at Appearance > Editor Access if you wish.
You can also select other menus to hide from Editors and Shop Managers (e.g. custom theme or plugin options pages), as well as hiding any elements on the Admin Bar.
Features
<ul>
<li>Allow access to Customize, Menus and Widgets for Editors and Shop Managers.</li>
<li>Theme Options remain hidden from these user roles.</li>
<li>Hide custom options pages from these users (if your theme/plugin has a custom settings page you wish to hide).</li>
<li>Hide any other menus or submenus. Simply select which menus you wish to hide.</li>
<li>Hide admin bar menus and elements. Choose which elements you wish to hide.</li>
</ul>

Editor Menu and Widget Access

Ideal

Allow and control Editor and Shop Manager access to the menus, widgets and appearance menu, plus other menus and adminbar items.

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.

Editor Menu and Widget Access vs Faster Appearance – Menus