“initialSpringVelocity: This controls the initial velocity of the animation. A value of 1.0 sets the velocity of the animation to cover the total distance of the animation in the span of one second. Bigger and smaller values will cause the animation to have more or less velocity.”
Excerpt From: “iOS Animations by Tutorials.” iBooks.
The bigger the value, the slower the animation will be, and the smaller, the faster it will occur. Hence a smaller value has a higher velocity. It sounds like the reverse in the text. In other words, if the velocity is set to 0.5, now the distance must be covered in half the time (a half second) therefore it would need to have a higher velocity to achieve that, and vice versa.