In general, Parallel bus is faster than serial because more pins/lines are available for transfer of data. So Ideally speaking a 8 bit parallel bus will be 8 times faster than 1 bit serial. Then how come USB (serial bus) is faster than any of its counter part parallel buses. one wild guess : Speed may be limited by interference and time skews of lines in parallel bus. This will not be an issue in serial bus. What are other major reasons?
I am wandering if this is a generic case then why did parallel bus come in to this world at all. In old days, we had only serial port on PC which supported few Kbps bit rate. That rate was not sufficient hence we invented parallel buses like PCI. Now we are again going back to serial bus (USB) for much faster speed. Then what is the use of parallel bus. I guess serial buses will also be less complex (few pins) and less expensive than their parallel counterparts. On what point does parallel bus beat serial bus?