Serial transmission is slower than parallel transmission given the same signal frequency. With a parallel transmission you can transfer one word per cycle (e.g. 1 byte = 8 bits) but with a serial transmission only a fraction of it (e.g. 1 bit). The reason modern devices use serial transmission is the following: You cannot increase the signal frequency for a parallel transmission without limit, because, by design, all signals from the transmitter need to arrive at the receiver at the same time. This cannot be guaranteed for high frequencies, as you cannot guarantee that the signal transit time is equal for all signal lines (think of different paths on the mainboard). The higher the frequency, the more tiny differences matter. Hence the receiver has to wait until all signal lines are settled -- obviously, waiting lowers the transfer rate.