Frequencies in atmega (uC)
The nrf24l01 SPI clock frequency is limited to 8 MHz (the max on-air data rate is 2 MHz).  If you are using hardware SPI, there is a divider that can be used to set the SPI frequency to a fraction of the processor frequency (CPHA register if you are programming registers directly, setClockDivider() in Arduino) allowing you to run the SPI clock at a lower frequency than the processor.