Is AI traffic harder on your CPU or GPU?

[Groundbound1]

This seemed like the most fitting place to post. (might be wrong)

So, when you wanna really crank up the AI in your flight sim, and you watch you framerates drop,  which is it that's working harder? Is it the CPU, because it needs to calculate all the positions, speeds, movements etc., or the GPU because it has to render all that stuff?

[Wii]

CPU because most AI is made (model) framerate friendly.