Understanding MobileSingleStickControl Speed Control (unity ver 5.1.3 f1)

I made a game on Android and I'm making final adjustments.
If I use the joystick above and move it too fast, I'd like to slow down the speed a little bit, what should I do?

By the way, it is described like this.

 vector.x = CrossPlatformInputManager.GetAxis("Horizontal");
vector.z = CrossPlatformInputManager.GetAxis("Vertical");