Real time compression and encryption needs ever faster processors, especially as data comm rates continue to increase. Inexpensive 16 bit CPUs at speeds of 40 Mhz are available today. 8 bitters passed 20 Mhz years ago. Raw clock rate specifications, though, mean little. Though the chip vendors are excruciatingly honest about specifying their clocks as real CPU bus rate, many designers still don't understand that the crystal frequency may be twice the bus rate.
Most processors divide the crystal by two.
That 40 Mhz crystal, then, may be driving your processor at a more sedate 20 Mhz. It's hard to build very high frequency crystals, so many of the speediest CPUs divide the input by 1. Some don't accept a crystal. Others multiply the input using a phase locked loop. This has two advantages.