This article will explain KeyShot's CPU usage
You may find that even with simple scenes, KeyShot is using 100% of the CPU.This is intended behaviour and does not depend on the complexity of the scene.
While KeyShot is rendering further samples, it will always use as many cores as you allow KeyShot to use, which by default is 100%. A simple Scene will not use less computing power, it will just render faster.
It is generally recommended to set KeyShot's CPU usage between 50% and 88% depending in the CPU in order to allow background processes to run smoothly.
You can limit the CPU usage in the top right of the Ribbon 
You can also set limits in the General Preferences to stop rendering after a certain time and/or after reaching a certain amount of samples.
This will stop KeyShot from rendering indefinitely.
