Introdution to Speed
Mario’s maximum regular running speed is 3 pixels/frame, or 3 speed. You always want to be holding 1 while moving left or right to run in NSMBW.
For reference, here are some of Mario’s speeds during different actions.
Running:: 3.0
Jumping: 3.0
Star Power: 4.0
Prop Spin (Up): 2.5
Prop Spin (Falling): 1.5

Pay attention to Mario’s X speed in this example and the rest in this guide (5-4).
Learning these is important for understanding speed optimization. The propeller suit may be convenient for vertical levels and some obstacles, but because of its slow speed while spinning, we want to utilize it as little as possible in horizontal levels.
This also shows the benefits of using the star power-up. More on the star and propeller power-ups soon!
In order to have speed, you must accelerate to that speed!
Downward slopes aid acceleration, so if you are trying to speed up, go down a slope. More on Slopes soon.
In a flat, open area (most levels start this way), in general, running is the fastest way to accelerate starting with no speed. Unless there is a wall…