Does Smooth shading cost more computing resource than the Flat one?

I always want to use the Smooth shading because it can make my model look better… But I worry about it will cost more computing resource. Is my worry true???

What are we talking here? OpenGL (=viewport) performance? Memory usage? Render times?
I think it’s safe to assume that smooth shading will have impact on all of the above. The only question is: How much of an impact?

I daresay the performance hit of smooth shading should be negligible on a reasonably modern machine. Why don’t you experiment a bit on your hardware and see for yourself?

I think you can rest assured not worry about smooth shading, that is just a fake shading trick.

Adding a sub-d modifier to make it truly smoother is a different story. With each view level the requirement increases.