To set CPU power at its lowest (perhaps to stretch battery life), use `cpufreq`.
To install this:
```
sudo apt-get install linux-tools-$(uname -r)
```
And then to use (as root)
```
cpupower frequency-set -g powersave
N=4 # number of cores
for s in $(seq 1 $((N-1))); do
  echo 0 > /sys/devices/system/cpu/cpu${s}/online
done
```
and use
```
echo 1 > /sys/devices/system/cpu/cpu${s}/online
```
to enable a core.

If editing code with `vim`, I can happily work in this lowest power mode on a Thinkpad T450, 
even under Kubuntu. On the other hand, as soon as you load a heavy website in Chrome
(or an electron app), the CPU restriction becomes obvious.