My folder usually hits the road at 35-40 pounds when I add the daily gear and when I was commuting 30 miles (one way) my rolling time was usually under 2 hours... this was the same speed I would see on my touring bike and touring worthy hybrid over shorter trips. Both curbed out at around the same weight and have similar aerodynamics.

The same trip on my road bikes is a little quicker by virtue of even better aerodynamics and and taller gearing to exploit that... this is what you need to take you past that 30kmh / 18 mph wall.

Most fit riders can manage 17 - 18mph on any decent bike, getting past that requires a lot more dedication and being able to ride 25 miles in an hour puts one in rarer company... the energy required to go from a speed of 17 to 25 mph requires almost twice the energy to overcome wind resistance.