Underclocking
Capping CPU frequency on Linux with cpupower: where to find it, the config file to edit, and the frequency-info and frequency-set commands.
Program you're gonna need: cpupower. It's in the official repository.
More info: CPU frequency scaling on the Arch wiki.
You can probably figure out the rest on your own...
Have a look at: /etc/default/cpupower
$ cpupower frequency-info
$ cpupower frequency-set -g powersave
$ cpupower frequency-set -u 3.1GHz
$ cpupower frequency-set -u 2.8GHz