Not so much that as Darkest Hour was actually optimized, HoI III is basically just band-aided over with "add-ons" (paid-for patches, as I call them, as without them the game is basically unplayable).
There's still some aspects I like about HoI III, such as not having to micro-manage Fronts you could care less about (or, as I like to do, create an army and only attach planes to it, then set it to the A.I., as I hate having to manage them), but it just feels barren. The "power-triangle" for Influcence among nations is a good idea, but it looses it's flair when the U.S. can easily be drifted into the Axis as a result. I also like how you can Amphibiously Assault any province, though the AI seems to never exploit this fact (they only invade a province if it has a port).
My biggest issue is that the the graphics aren't impressive enough to constitute the 3D map; if they incorporated what they had into a faster 2D game and kept some elements from HoI II (such as being able to view foreign ministers), kept the HoI II Naval system (though I do like how they treated carriers), I think I'd be a good game.
I really don't think a simulator like this needs a super-3D game, as such games are more about the speed then the graphics (the tactics and their results are all in your head, after all).