Install powertop. It will allow you to tune the power consumption.

The biggest impact would probably be changing the CPU scaling governor. This controls how the kernel scales the CPU frequency depending on the work load. It's per-processor core. An application like tuned can do this for you and make other adjustments automatically, depending on the profile you select.

Most distributions use the ondemand governor. To see available frequency governors you can do something like:

cat /sys/devices/system/cpu/cpufreq/policy0/scaling_available_governors

and change it with:

echo conservative > /sys/devices/system/cpu/cpufreq/policy0/scaling_available_governors

Here's a link to kernel docs explaining each one.

There is Powertop by intel which could help reduce energy usage with Intel CPUs. Otherwise, I know of TLP which is mainly aimed at laptops to reduce battery usage.

There is intel-undervolt which might already do what you need.

There is also the cpupower utility which works with the pstate driver which allows you to tune the frequency.