Ditto the suggestion to keep the Conti GPs and try latex tubes.
I've tried several road tires in 700x23 to x25, with rolling resistance rated from 12-14, and can't say I felt or measured any consistent differences. Getting more aero made more difference. But latex tubes feel much better and are a little bit faster or demand a little less effort for the same speed.
Mostly I switched to latex after all our roads were repaved with the worst, coarsest chipseal I've ever seen. It's like the just glued a bunch of railroad ballast gravel together with epoxy. Latex tubes and decent tires made it pleasant to ride again.