Alright, here goes for my AC66U:
admin@RT-AC66U:/tmp/home/root# nvram set clkfreq=600,300,150
admin@RT-AC66U:/tmp/home/root# nvram get clkfreq
600,300,150
admin@RT-AC66U:/tmp/home/root# openssl speed aes-128-cbc
Doing aes-128 cbc for 3s on 16 size blocks: 1823182 aes-128 cbc's in 2.99s...