I read iPhone CPUs are usually underclocked, it does happen when you have modern compact devices, like the MBAir for example, so I guess, especially if unlocked/pawned could sometimes result in a "normal" CPU speed of 620 MHz..
does it actually harm the iPhone? it probably just makes it heat up eventually more and the battery could last less
it's just a supposition, cause having iPhone's ARM CPU just randomly overclocked should be more dangerous and problematic