http://brandonoxendine.com/writing/why- ... -its-fixed
...I'm not sure what kind of device you're reading this article on, but chances are you don't have an endless amount of processing power. Say you have 100 photos, animated GIFs, or videos in your "scroll history." These kind of assets all stacked up on a single page can slow down any upstanding browser, no matter the power of the CPU. And I'm not so sure browser crashes are good for user experience.