ZDNet.com is one of those sites unless you visit with the Tracking Protection mentioned above enabled. Their site is heavily laden with active ADs and script which are completely unnecessary for viewing the content, such as it is. However if you want to leave a comment you will have to unblock all that. Id expect if you when there and observed the kind of garbage they are pushing out you would have a pretty good idea of what's causing problems on other sites. There's another site I have seen which appears to have converted to almost everything to client side scripts (AJAX or similar) which is horrendously inefficient as implemented (proof you can implement bad code using any technology) and will near constantly run high CPU utilization.