I/O is definitely the slowest among the fundamental operations of a computer. Accessing the RAM is in the order of nanoseconds (10e-9 seconds), while accessing data on the disk or the network is in the order of milliseconds (10e-3 seconds). For the bandwidth, it is the same story; RAM has a transfer rate consistently in the order of GB/s, while disk and network varies from MB/s to, optimistically, GB/s. I/O is usually not expensive in terms of CPU, but it adds a delay between the moment the request is sent and the moment the operation completes.
