In some of the levels, your character starts directly in front of a wall; you have to do a jump and a wall-jump to switch directions. You can do this fast enough without wasting any time, by pausing and jumping at the exact same time, at the first frame when the level loads. (Pausing can be done with the 'P' key)

You know you do this right, if you manage to pause the game when the level timer shows 0.016, you hear the jump sound, and the character is facing away from the wall. After that, unpause the game and jump one frame after that, to execute the wall-jump. This is the fastest way to do a wall-turnaround in those levels.