What is the fastest CPU speed?

Fastest production clock speed I know of, is a CPU on an IBM Z series mainframe computer. 5.5 Ghz. Fastest ever clock speed, using extreme overclocking techniques, was over 8 Ghz. In that sense, because it was AMD, AMD actually built the fastest CPU known to mankind, but only if you allow for what can be done with overclocking.
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https://www.quora.com/Which-is-the-best-and-fastest-processor-in-the-world
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