Using curved lines instead of straight lines in an engine

I've noticed that 3D engines tend to have more difficulty rendering curved lines (as in, it takes longer). Why does it use straight lines?

My Idea:

Have the engine use all curved lines, a curved line can be a straight line (i.e. a curved line with no curve) but a straight line can't be a curved line. Wouldn't this allow the engine to perform faster?

Correct me if I am wrong, but, what I'm saying is: Would it be better if the engine us开发者_如何学Ced curves instead of straight lines?