Thats odd, its never happend to me. Maybe you guys should try Opera.
On Firefox (3.0), never happened to me either.
Maybe its one those add-ons I don't know.
Btw, regarding what Thrawn said, those slowdowns can be intensified if you have too much add-ons, specially the ones that are visually present like this Link. (Screenie turned out bad, just zoom in the upper left corner)