I have a friend, who is a strong rider, but because of her body type is limited in the power she is able to generate and thus cannot go much faster than 17 - 18 mph. She is very tall and very lean, has little or no muscle mass. Great for climbing or endurance rides but tough on performance. This is why some cyclists specialize in climbing, or sprints or GC.

Her trainer's recomendation is that she build muscle mass. That means hitting th gym and lifting weights. Need to do leg lifts, calf pulls, etc. Muscle equates to power. Power equates to speed. It's as "simple" as that.