Apple II for intro to microprocessors

I can recommend you consider that the difference between the fastest speed and the optimal speed comes down to a number of things. Consider one case, wherein we speeded the operation of a multi-computer security system by reducing the baud rate of the monitor terminals from 9600 to 1200. That reduced the amount of idle (ENQ/ACK) traffic by 90%. Moreover, lots of I/O tasks require too much speed if you have to test a flag before fetching each byte. However, if your processor is running at a rate synchronized with the transfer, you only have to check the flag once, then use a loop tuned for the exact length of the transfer cycle. As Tony points out, it's based on the externally imposed rate, and if your oscillator doesn't synchronize with that, you have problems.