shawfactor

If you are sending large volumes of emails this is the plugin you need.
Generally wordpress emails are best sent by a third party, like gmauil or Sendgrid. But connecting and sending a large number of emails to third parties will slow your visitor experience down.
Unless those emails are sent in the background. This plugin does that.

LH Email Queue and Log

LH Email enables you to queu your emails so they are sent asynchronously.

kylereicks

Head.js is a script to asynchronously load and manage dependencies of javascript and CSS assets.
This plugin loads the scripts and styles enqueued in footer with the head.js loader. It does not include head.js’ feature detection.
Head.js.wp assumes that scripts are loaded via <code>wp_enqueue_script()</code> and that any script enqueued in the <code><head></code> intends to be loaded before the rest of the page.

Head.WP

Head.js is a script to asynchronously load and manage dependencies of javascript and CSS assets.

LH Email Queue and Log vs Head.WP