Is the bike a good fit for you and set up properly? A bad seat height could really impact your ability to put power down.
After that I'd get the bike serviced and make sure everything is running smoothly, a stiff bearing or badly adjusted gears can really slow you down.
Next I'd maybe look for smoother rolling tires - basically something that looks smooth rather than knobbly, and at a suitable pressure (higher pressure is a bit faster but less comfortable).

Beyond that you're looking into getting lighter bikes (probably not worth it) or improving your fitness and cycling more. . An average cycling speed on road bikes is about 12mph so you're not that far behind the curve.