Does Thrust or RPM the main contributor of speed for a multicopter?

What do you think is the main contributor for speed for multicopter?
My quad copter flies at cruising speed of 20km/h. I like to increase to 40km/h, do i can a high KV faster RPM motor or a low KV but bigger props and higher lifting power motor?

State more details of what you have now, and supply a .bin log file