There's no real way to determine the "average clock speed" of a CPU, unless you can account for every microprocessor ever made. Microprocessors, including those used for central processing units, have ranged in speeds from 500 KHz to 6 GHz.
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The fastest consumer processors on the market today support clock speeds up to 3.2GHz. However, this is not the sole determining factor of performance. Processor architecture is another factor. For example, the NetBurst architecture used in Intel's Pentium 4 processors was so inefficient that a modern single-core processor running at 2GHz can easily outperform a Pentium 4 running at 4GHz.