Disqus is very popular (and expensive for the Pro version) but it can really slow down you site. There is a little trick very useful called Disqus DCL. This little script loads Disqus only when the page scrolls to the comment area. This way Disqus doesn’t load when unecessary. Visit Disqus Conditional Load by Joel James. For more details on installation intructions check out this post on Crunchify.