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.

nigro.simone

This small plugin (5 Kb) loads the stylesheets asynchronously and improve page load times.
The recommended use of this plugin is to load your vital stylesheets synchronously and non-vital CSS files asynchronously. 
Non-vital CSS-files can be for example: fonts, icons, before the fold template-specific CSS, etc.
You can choose which files to load synchronously with a filter in your function.php, eg.:
<pre><code>// exclude main and child stylesheets from delivery optimization
function exclude_from_delivery_optimization($handle){
 return in_array($handle, array('main-stylesheet', 'child-stylesheet'));
}
add_filter('speed-up-optimize-css-delivery', 'exclude_from_delivery_optimization');
</code></pre>
Note: this only works if your other plugins and theme add the CSS correctly.

Speed Up – Optimize CSS Delivery

This plugin load the stylesheets asynchronously and improve page load times.

LH Email Queue and Log vs Speed Up – Optimize CSS Delivery