Modern CPUs from AMD and Intel both use a boost feature like this to make better use of the CPU when some cores are not heavily utilized.
This is the way they are meant to work. Disabling the feature will only slow performance.
Are you seeing any of the cores at 100% during your game?
If the CPU is not hitting 100% in at least one core during the slow down, the issue is more likely that your settings are too high for your graphics card.
If the CPU is regularly reaching 100% in one or more cores during the game, you may just need a better CPU.