Rotational mass is poorly understood, and doesn't have as much overall effect on speed as people assume. Heavier wheel rims are a bit more difficult to accelerate, but once they get up to speed they hold their speed a bit better than lighter rims. So when you say it <i>feels</i> sluggish, perhaps you are feeling that lag when you try to pick up speed, and not noticing that you coast a bit better once you get going.

Also are the tires slick or knobby? I think that makes a big difference.