cpu clock too low / force cpu clock / overclock
That sounds most probably like an old Mac, which will not go very fast anyway these days. 

But the symptoms really can describe a few things of underlying problems.


misconfiguration: look at the programs installed and delete what's unneeded, especially background applications (make a clean install ot the OS if all else fails)
SMC settings got confused: perform an SMC-reset
aging hardware: the thermal grease could be bad by now and might need replacement (try the other options first)
being dirty: if the grills for the fans are clogged (most probable here) just open the case, clean it out.