That's not extally true, because 99% of the time you are not running JUST one program. There is always something on the background. This slowdown that you hear about occurs in games, and its is because of the greater overhead on the memory bus. Having two processors accessing it, slows things down. However, if you have a few programs running in the background (anti-virus, firewall, chat) single cpu programs can run faster than on a single cpu system.