I run Firefox in a Linux VM for the purpose of visiting potentially malware-infested websites, and I've noticed that simply scrolling the webpage is enough to send host CPU usage up to 70-80% on a Core i5-3337U. I then repeated the test on a desktop computer with a much more powerful Core i7-2600, and scrolling up and down was enough to consume 25% of the host CPU - 2 whole cores.
Why does it take so much CPU power to scroll a webpage?