It's using an IPP function - 
#ifdef USE_IPP
    int myfreq;
    ippGetCpuFreqMhz(&myfreq);
    fprintf(stderr,"CPU real speed: %d MHz\n\n",myfreq);
#endif
This is what I inserted into the code. No idea how heavy the overhead for that function is, but I believe it's very small.
Regards,
Simon.