Click around and see how fast this page responds!
"Websites may be much busier under the hood than they appear on the surface"
If you click around on various links, you may notice a small delay in how long it takes before the page acknowledges your click. (This is something we've worked hard to optimize in EdgeHTML 15!)
Even after the site "settles," you may note that CPU usage still remains fairly high, because the page is doing a lot of work to monitor the position of ads, the position of the mouse cursor, loading asynchronous content, etc.
If you look in the console, you may see several logs giving you an idea of the kinds of things such sites can do, which may have a detrimental impact on responsiveness, especially if there are many tasks queued simultaneously.