stapfam

OK- What bike- What setup- what components?

Should not matter a great deal but if 15mph is your max on the flat- then eliminate the obvious. Brakes binding- Low tyre pressures- Tight bearings and many other things can be slowing you down. Then look at the weather, Headwinds- high humidity or temps can affect you. Then look at yourself. Are you trying to go fast before you have warmed up-Have you enough fluids in the body- enough salts- enough carbs.

After that look at what you want to attain and is it feasible. I rarely get above 18mph on the flat. If I do it is with a tailwind and I will struggle on the way back. To build speed above that takes training. Plenty of useful miles where interval training will take place. Plenty of hills where I can build the legs and lungs. Then there is cross-training to work the muscles that cycling will not improve but are necessary to get bike fit. And I still may not improve speed because I can see no point for my type of riding to just going faster. What I will do is make myself stronger and build endurance for the rides I like to do.