The rendercows work in the background in a low-priority state. It means that if you don't run another program that needs itself a lot of CPU usage (like a 3D game), the cow will use every CPU cycle available. But when you temporarily need the cycles for a foreground application, the cow will keep a low-profile and use minimal cpu power.

It is the best way to deal with background rendering. In some other applications the render nodes eat always 95/100 % CPU power and slow down the entire machine no matter what. The drawback is, with Vue rendercows, sometimes you don't even remember they are working :)