P-Speed

Walking speed
Running speed
P-Speed
Walking and running speed are pretty intuitive, but P-Speed is reached by filling up a meter by going Running Speed
(And as it doesn't get reset to 0 instantly you can drag out the filling of the P-Meter for some time)

There's also ways to go beyond the p-speed cap in smw, for example sliding.
However when you press the directional pad it resets that speed, so the way to keep it is by not pressing any directional keys and keeping yourself away from the ground.

P-speed refers to the boost in Mario's running speed after running forward for a period of time. The name comes from the P-meter bar first used in Super Mario Bros. 3; though no bar is visible in SMW, it functions very similarly. The meter increases as Mario runs on the ground, and decreases while in midair or turning. Once the meter is fully filled, Mario will gain P-meter and start sprinting, and if he has a cape, he can jump to start flight or spinjump with an extra large jump. Since it boosts Mario's speed significantly, you usually want to build it as soon and as quickly as possible.

The P-meter has a few quirks that aren't immediately apparent, however:

If you jump with P-speed, the P-meter will basically function like you're still on the ground; in other words, if you turn, you can still rebuild the meter while in midair.
Gradual slopes are the only slopes that Mario can build P-meter while running up. Once he has P-speed, he can run up any slope (except very steep) without losing it.
The P-meter will decrease while flying, but not during the initial jump; only when he actually enters flight mode does it start.
The P-meter won't decrease while swimming or climbing.