The default speed of the motion.
A speed of 1.0 plays the animation at its original speed. A speed of -1.0 plays the animation backwards. A speed between 0.0 and 1.0 plays the animation forwards and slower. A speed greater than 1.0 plays the animation forwards and faster. For example, a speed of 2.0 plays the animation twice its original speed while a speed of 0.5 plays the animation at half its original speed.