Undervolting is where you instruct the CPU to run at the same performance but consume less power.
Undervolting doesn’t hurt performance, in fact modern CPUs are programmed so that when under load they will increase performance as much as possible while keeping the power consumption below a certain limit and not overheating. So reducing power consumption for the same performance means the CPU will respond by increasing performance to reach the same power consumption under load.
In total undervolting reduces power consumption for the same performance, and when hit with a full load it will ramp up to the same power consumption but higher performance.