MIPS is only approximate because some CPU instructions do more work than others, and CPU performance is also influenced by other factors like the amount of internal (cache) memory on the CPU chip, and the ability to execute several instructions simultaneously.
Furthermore, system performance depends upon much more than CPU speed. The speed of input, output and storage devices and the speed of transfer of information between memory and the CPU may be more important than CPU speed in determining the execution speed of an application. Memory and storage capacity are also important determiners of system performance.
Figure source: The Harrow Group.
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