Mike Jolley (a11n)

Testing your WordPress site? This plugin allows you to display memory, page load time and query count in the footer of your site on both the front-end and within admin.
This plugin is partly based upon the WP Query Counter by Steven Word (http://www.stevenword.com/plugins/wp-query-counter/) and has been customised for my testing needs.
If you’d like to contribute to the plugin you can find it on GitHub: <a href="https://github.com/mikejolley/WP-Page-Load-Stats" rel="nofollow ugc">https://github.com/mikejolley/WP-Page-Load-Stats</a>

WP Page Load Stats

Not sure why this only has 100+ users but this is a great tool!

Excellent!

Display memory, page load time, average load time and query count in the footer of your site.

nikdow

Use this plugin to display detailed information about your server’s OPCache, including memory utilisation, list of all scripts cached and a useful memory utilisation visualisation based on the file system tree.
This plugin is based on https://github.com/rlerdorf/opcache-status which is for general PHP use, we have turned it into a WordPress plugin for ease of installation. Apart from some rearrangement of scripts to suit WP load conventions, the code is to date unaltered. WordPress capability ‘activate_plugins’ is required to see it in the admin menu and to run it.
After you install and activate this plugin, look in the admin Tools menu for OPCache.

OPCache Scripts

Use this plugin to display detailed information about your server’s OPCache, including memory utilisation, and a list of all scripts cached.

WP Page Load Stats vs OPCache Scripts