Here is the best plugin you will ever need to load JS and CSS asynchronously, and improve Google pagespeed which in turn will help you rank better. Google uses pagespeed as one of the factors in ranking so it is in your best interest to have a website that has Asynchronous loading scripts and especially CSS.
Converts render-blocking CSS and JS files into NON-render-blocking, improving performance of web page.
Async JS and CSS is a plugin that by default does a great job, but you should consider using the same settings as I have used above. You can also use http://www.cleancss.com to compress and clean your CSS, so you can get even better page load speeds. Add Hyper Cache or Incapsula to all of this and you will have one of the world’s fastest loading WordPress site.