Demigod is supposed to run much better than Supcom. Supcom has a problem because high unit counts and physics = lots of work for your computer. Demigod has fewer units so it should lag less.
Yeah. HOWEVER Demigod has higher resolution textures, more shader effects, more particle effects, and mesh deformations(the bending with animations and such, rather than rigid bodies like in supcom) which all hurt performance.
So it's a give and take.
But the big difference I imagine is in Demigod you can turn down the texture resolution in options, turn off the shader effects, turn down the number of particles(i'd assume they have this too, simplify the animation quality(also assuming, many games have this but some don't).
In supcom you can't turn down the number of units or projectiles.(well yuo can play 2vs2 and smaller games, or less max units 
But that effects gameplay, not just a setting.) The things that lag Supcom a lot can't be turned down.
So it's probably more that Demigod runs better on low end machines, but you can turn it up to look beautiful and poop on your high end machine.
