In common PC computers, the system memory is usually much much faster than the peripherals and PCI bus. Depending on the particular PCI clock rate (33, 66, 100, or 133 MHz) and bus width (32 or 64 bits), PCI can give you several hundred megabytes per second. However, modern PC memory can go several gigabytes per second.
The PC motherboard has a bridge chip that exchanges data between the CPU's peripheral bus and the PCI bus, and provides the necessary bus handshake and arbitration.