These days, most machines come with advance power saving options. To save power, machines reduce cpu speeds whenever it is not in great demand. Even though the cpu may have a specified speed of 2 or 3 GHz, these speeds are reached only when there is enough load on the system and when full processing power is needed.
This is very important to remember which discussing or measuring system performance. Since the performance can drastically change with cpu speed, cpu speed must be held constant while doing any performance measurement. Typically, you will need to switch of the dynamic cpu speed revving by setting appropriate flags in the power-saving setup files.
Measuring maximum performance of a system or comparing performance of two software products is meaningful only when cpu power is held constant.