Unity3D Slerp旋转速度

我正在使用Unity3D.我想旋转一个物体面向鼠标指针的方向,但允许最大旋转速度,如"每秒最大100度".

文档中有一个例子,但它没有做我想要的.
我认为Time.time应该是Time.deltaTime,我无法真正理解最后一个参数的作用.它应该是与起始向量相加的数字吗？
http://docs.unity3d.com/Documentation/ScriptReference/Quaternion.Slerp.html

另外,我无法真正理解最后一个参数的作用.这是轮换的时候吗？

我正在使用的代码

Plane plane = new Plane(Vector3.up, 0);
float dist;
void Update () {
    //cast ray from camera to plane (plane is at ground level, but infinite in space)
    Ray ray = Camera.mainCamera.ScreenPointToRay(Input.mousePosition);
    if (plane.Raycast(ray, out dist)) {
        Vector3 point = ray.GetPoint(dist);

        //find the vector pointing from our position to the target
        Vector3 direction = (point - transform.position).normalized;

        //create the rotation we need to be in to look at the target
        Quaternion lookRotation = Quaternion.LookRotation(direction);

        //rotate towards a direction, but not immediately (rotate a little every frame)
        transform.rotation = Quaternion.Slerp(transform.rotation, lookRotation, Time.deltaTime * rotationSpeed);
    }
}

我认为弱点位于Slerp的第三个参数中,但我无法弄清楚要放在那里的内容.