Reading of the flash is independent of CPU frequency. At low frequencies, the time spent on reading a flash cell is less than one clock cycle, meaning that the flash is only enabled for the time it takes to read, a feature found on all AVRs. This is another important contribution to decrease the overall current consumption of the AVR.  Flash sampling allows the application to turn on the flash array, read instructions and then turn it off again. The figure shown here is the ATmega644P with and without Flash sampling running at 3V.