Avram

Ultimate debug bar for your WordPress website. On each page of your website a neat debug bar will be displayed with the following info:
<ul>
<li>PHP version</li>
<li>Request execution time</li>
<li>Request memory consumption</li>
<li>WordPress-related PHP constants defined in your wp-config.php file</li>
<li>All website options (wp_options)</li>
<li>Request data</li>
<li>Timeline (waterfall) where you can see which portion of execution takes too much time</li>
<li>List of all hooks triggered on the page whose execution time exceeds 1ms, along with some stats</li>
<li>List of all database queries performed on the page, along with their source and execution time</li>
<li>Last 100 lines of your debug.log file (if it exists in wp-content)</li>
<li>List of loaded PHP files (categorized)</li>
</ul>
Besides that, it will track request data/hooks/db queries/mem. consumption in AJAX calls too (beta).
This plugin is meant to be used during development and/or debugging only. Only administrators and network administrators (in multi-site environment) can see the debugbar, unless the <code>WP_DEBUG</code> is set to <code>true</code>. In that case, everyone will see the debugbar.
Try it instantly on <a href="https://tastewp.com/new?pre-installed-plugin-slug=ultimate-debugbar&redirect=%2F&ni=true" rel="nofollow ugc">tastewp.com</a>!

Ultimate DebugBar

Ultimate debug bar for your Wordpress website.

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.

Ultimate DebugBar vs Faster Appearance – Menus