While there is no hard documentation on the minimum CPU speed (clock speed) requirements, it is somewhat difficult to find a CPU with less than 2 GHz nowadays. This low watermark seems to be about the minimum required for Elasticsearch to avoid problems.
Anything above 2 GHz will perform acceptably, even with a single CPU; this is adequate for testing purposes. For production environments, look for CPU clock speeds above 2 GHz or 2.3 GHz to avoid problems.