The invention introduces a non-transitory computer program product for adjusting operating frequencies when executed by a processing unit of a device, containing program code to: collect an interface-activity parameter comprising information about data transmissions on a host access interface and/or a flash access interface; select one from multiple frequencies according to the interface-activity parameter; and drive a clock generator to output a clock signal at the selected frequency, thereby enabling the host access interface and/or the flash access interface to operate at an operating frequency.