I paid $30 for an old Bridgestone 400 with triple-butted Cr-Mo main tubles. Swapped wheels + cassette ($43). Swapped brakes ($15). I can cruise all day betweeen 17 and 18mph on a calm day without breaking up a sweat. 20mph on flat land is doable for 4 hours as long as there is a constant supply of water to stay hydrated.

Noobs believe that spending big bucks on a bike will instantly result in a big jump in speed. Put a 10lbs bag of rice in your backpack and go out for a ride. I doubt that you'll see more than a 1mph drop in cruising speed. Focus on riding technique and the motor.