On Intel based systems, you can usually run the following as root

echo performance > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor

If you have more then one core, which is likely the case on a contemporary machine, repeat the command for each. You can find out the number of cores with

ls /sys/devices/system/cpu/ | grep cpu[0-9].

To get back to the default poower level, run

echo ondemand > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor