Flat Shading vs Smooth Shading (smoothing groups)

Not sure what to believe as I keep finding conflicting answers on the net and even on here. Wondering if someone with experience in the matter can give some input.

When exporting meshes, you have the option to include the flat or smooth shading on the base mesh. I’m looking for whatever is less taxing on processing. I see people saying that going for low poly with flat shading is worse for the GPU due to the way smoothing interpolates space between vertices, yet I also see thing like this (Shading - Wikipedia) saying that “smooth shading is more computationally expensive”.

Which is true?

I’ve decided that I don’t care.