Older mainframe tape drives used to run at 75 ips (inches per second). The data was recorded at a density of 1600 bpi (bytes per inch). Due to blocking, data was never recorded in a solid, continuous stream on these tapes because flaws and imperfections would make it dangerous to count on the tape being perfect over a long stretch. So these interblock gaps on the actual tape would give the CPU a short breather; these gaps are dead space on the tape, places where no real data exists.

Pretending that the tape is continuous, that there are no interblock gaps, 75×1600 means 120,000 bytes per second coming out of the tape drive. This sounds like a lot, but taking its reciprocal, this rate is 0.0000083 seconds/byte, or 8.3 microseconds per byte, or 8300 nsec per byte. If the computer is performing one million instructions per second, which is quite a slow rate nowadays, it takes one microsecond per instruction. Thus the tape drive is more than 8 times slower than the CPU and a lot of the CPU's time would be spent vainly checking the data ready bit and finding it to be 0.