CPU's slow down with time when run for extended periods at elevated voltages and (to a lesser extent) temperatures. It's an effect called hot-electron gate impact ionization - AKA "hot-E". The "hot" does not refer to temperature but refers to the energy levels of the electrons.
It's a pretty esoteric subject, but for on-the-side reading, google "hot electron gate degradation", or something similar. It will turn up things like
this.
The summary though is that CPU's slow down with time. Increasing the voltage increases the effect substantially. Increased temperatures also have an effect. One of the reasons why CPU's overclock is that manufacturers set aside "slowing down" margin for the CPU's.