When I first started commuting to work I rode a 35+ pound single speed Sun Cruiser with high handlebars. The positioning (aerodynamics), the weight (inertia), the cruddy drive (loss of efficiency), and wide heavy tires (high rolling resistance and inertia), all added up to a slow and exhausting ride.

I switched to a single speed track style bike that had 23c tires and was 11 pounds lighter with a nice new drivetrain and drop bars, it was the difference from slow and exhausted to relatively quick and slightly tired.

A lighter bike with low rolling resistance tires will make a difference, especially if you're in the 30+ pound range dropping down to low 20's.