According to my boss (who does some Flash development) it would seem the particles in the advertisements are to blame for higher CPU usage in Flash animations.
With over ten tabs open I have been able to peg both of my cores at 100%. It isn't abnormal for me to leave 3-4 IE windows open with 3-10 tabs open in each (not all 3-4 IE windows are hardforum.)
Edit: There's something really wrong with the ad. When I went to end process on one window I left on at home with 9 tabs of hardforum open, it was taking up over a gig of memory. Classic infinite loop and memory leak behaviour?
Im not sure if it is the newegg ads fault, because I didn't have a problem till I updated firefox, and after reading other posts on the web it seems that the .10 and .11 updates cause abnormally high cpu usage.
Pretty sure i've also been having problems with their ads, occasionally, for a long time now. Slow page loads + high cpu usage, both IE and firefox. It is a convenient way of clicking-thru to Newegg though