As I mentioned in the other comment, perhaps the problem is poor Linux support on the Unity side of things? Which would be... how shall we say... typical, since Unity also targets mobile platforms. You'd think they'd allow developers to properly optimise for fan noise.

Regarding DD: first, it's also implemented in Unity, so... but that aside, even if the whole screen gets repainted, it shouldn't max out. The graphics are just not demanding enough for that.