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.

Paris Holley

This plugin is meant to be a drop-in to your wordpress installation with no additional configuration. The goals/features of this plugin are:
<ul>
<li>Load javascript files in an asynchronous manner to improve time to DomReady/OnLoad</li>
<li>Use existing wordpress APIs for backwards compatability and prevent coupling to this plugin</li>
<li>Leverage dependency model wordpress provides for assets to improve loading performance</li>
</ul>
Please submit bugs or contributions to the github location and not here on wordpress’ system:
https://github.com/parisholley/wordpress-asynchronous-javascript/
SEO: async js, asynchronous js, async javascript
<h3>Incompatibility</h3>
This plugin will not work out the box with the following plugins (unless they are modified to support asynchronous loading).
<ul>
<li>Jetpack by WordPress.com (social plugin)</li>
<li>WP Most Popular</li>
</ul>

Asynchronous Javascript

Improve page load performance by asynchronously loading javascript using head.js

LH Email Queue and Log vs Asynchronous Javascript