The speed of the CPU is measured in gigahertz (GHz). Memory chip speed is measured in nanoseconds. A nansecond (ns) is equal to one billionth of a second. When adding more memory to a PC, it is best to match the speed of the existing memory chips. If faster memory chips are added to an existing memory chips. If faster memeory chips are added to an existing slower memeory chip bank, the newer faster chips will run at the same speed as the existing chips. The chips will run at a slower speed, they are not cost effective. Another consideration is the maximum speed that the motherboard system bus can support as well as the chipsets. You must check the PC, chipset, and BIOS documentation before attempting to upgrade RAM. This is especially true if the PC you are attempting to upgrade is ancient lol. While faster RAM will slow down to run on a computer with a slower bus speed, RAM modules with a speed slower than that of the motherboard bus will not run.