I recently had some fun overclocking my old i5 4690.

IIRC, running the base frequency at 3.9Ghz instead of 3.5GHz, yield a very modest performance boost but added 20% more power consumption and temperature.

I then underclocked it to 3.1Ghz and the thing barely ran at more than 40°C under load and power consumption was super low! The performance was more than mediocre though...