Greg Linden has a great run-down of a Google anecdote from Marissa Mayer:
After a bit of looking, Marissa explained that they found an uncontrolled variable. The page with 10 results took .4 seconds to generate. The page with 30 results took .9 seconds.
Half a second delay caused a 20% drop in traffic. Half a second delay killed user satisfaction.
Even in this day of broadband, fast matters.