More and more sites are making use of complicated javascript code that needs to run when the page is loaded. Some sites do this in an unobtrusive manner, some stomp all of your browser as you have to wait for a minute or more as the javascript "does its thing".

The post on this site (http://betterexplained.com/articles/speed-up-your-javascript-load-time/) provides a bunch of tips that you can use to improve the pageload time of your javascript so that you don't fall into the wrong category of site.