Parallel buses are obviously much faster than serial buses. For example, if we used a serial bus to transmit bytes, it would take 8 times as long to transmit a byte this way than with an 8-bit parallel bus. Thus, most internal buses in a computer are parallel.

However, buses that connect I/O devices to a computer are serial. Most microcomputer manufacturers use such low cost buses to connect slow devices like printers and keyboards to computers. The time it takes for a keyboard to transmit a byte is significantly less, even using only one data wire, than it takes for the mechanical key to spring back into position, and users never know the difference. However, disk drives can often transmit data at very high speeds, so a parallel bus usually connects them to other components like memory and the CPU.