Odd if it works well on ipad and mobile but not on a desktop because each device loads the same code and scripts. How does the demo site for Preferential work for lag for you? http://demo.styledthemes.com/preferential/
Also, are you using Jetpack? Some people have posted in the jetpack support forums that they are getting slower websites when Jetpack is installed. Do you have a link to your site that I can see and test?