Can I use the published clock speed in MHZ of the processor reliably as a relative speed rating?

For back-of-the-envelop capacity planning, it is nice to have a relative speed rating for various processors. You would like to be able to say with confidence that a given processor-bound workload running on machine A running at 400 MHz will execute in 1/3 the time on processor B running at 1.2 GHz that is 3 times faster.  For the most part, so long as you stay within the same processor family, you can do that with Intel processors.

Benchmark results consistently show that within a processor family, the performance of an Intel processor usually scales linearly with clock speed (in MHz), all other factors like cache size and bus speed being equal. Figure 3 below shows a chart which illustrates this point. Four representative sets of published benchmark results are plotted for Pentium II processors in the range of 300-450 MHz. That performance scales linearly as the clock rate increases is evident. Within a processor family, it is reasonable to expect a Pentium IV at 1.8 GHz to run roughly 50% faster than a Pentium IV 1.2 GHz box.

Figure 3. Within a family of processors, performance of Intel CPUs scales linearly with clock speed.

However, if  you try to compare machines from different processor families, you are apt to find that while clock speed is still important, there are other architectural features that matter. For instance, the 386, 486, Pentium, and Pentium Pro machines represent four different Intel processors families, the P3, P4, P5 and P6, respectively. The P4 introduced instruction pipelining to the Intel processor line, the P5 uses a dual integer pipeline (also known as a superscalar architecture), while the P6 features a highly parallel microarchitecture design. Running at similar clock speeds, P4, P5 and P6 machines will show markedly different results.

Since the advent of the first P6 Pentium Pro machines, subsequent versions of the Pentium II, III and IV are all P6 family machines with a similar internal microarchitecture. Machines within this processor family can be expected to scale roughly as a function of clock speed, as illustrated above. (In fact, Intel tweaked the internal architecture of the Pentium IV chip specifically to help it scale linearly with faster clock speeds.)

More recently, Intel has built multiprocessor chips where on-board CPUs have the capability to run at different speeds. Whenever higher processing requirements, these CPUs have the ability to execute faster — in an “over-clocked” mode, which also requires proportionally more electrical power. Windows Counters in the Processor Information counter set provide insight into this behavior.