Momentum preservation

One problem with current SRB2 gameplay is that players instantly lose all extra speed any time they are running on the floor, unless they are not rolling. This also prevents characters from gaining extra speed from running down slopes, meaning that the only way to do so is, again, by rolling, therefore making the experience for no-spin characters far more gimped.

This happens because the friction in the game is constantly holding back your momentum that's applied to it every frame. Hence why your speed is kept midair, because there's no friction in the air (and also why you can keep your speed on ice, because of low friction). The P_3DMovement and P_2DMovement add momentum the player character when pressing movement keys, but only if they're below max speed, and thus causes players to quickly decelerate if they're past that.

So the trick is to tell the game "no no -- this is our max speed" to prevent the speed loss from occurring.

Edit: To avoid confusion, let me explain what the ideal solution would be. The best approach would be for characters to gain or maintain speed in the same way they do when they roll on a slope, except making rolling not required for this momentum build/preservation.