tested the mhz tool on a vim2pro (with performance cpufreq governor, cpufreq-info reporting 1.51Ghz on cores 0-3, 1000 Mhz on cores 4-7), here’s the result:
root@vim2p:~# taskset -c 0 ./mhz
count=645643 us50=22806 us250=114044 diff=91238 cpu_MHz=1415.294
root@vim2p:~# taskset -c 7 ./mhz
count=413212 us50=20676 us250=103364 diff=82688 cpu_MHz=999.449