slow rotation over time?

Hi, I have a spaceship that flys around until it finds a target, trying to slow down the rate in witch it turns…
everything i have fun online tells me to use:
transform.rotation = Quaternion.Slerp(this.transform.rotation, rotationA, Time.deltaTime * TurnSpeeed);

however no mater turn speed is 0.1f, 1, or 1000, it dose not make it slower.
transform.rotation = Quaternion.Slerp(this.transform.rotation, rotationA, Time.deltaTime );
by its own is closer, but its still not turning slow enough, if their a more efficient way to slow this down?

if (Newdirectionflighttimer < 1) {
					randomDirection = new Vector3 (Random.value, Random.value, Random.value);
					Newdirectionflighttimer = 12;
				}

				Quaternion rotationA = Quaternion.LookRotation(randomDirection);
			transform.rotation = Quaternion.Slerp(this.transform.rotation, rotationA, Time.deltaTime );

				Newdirectionflighttimer -= 1 * Time.deltaTime;

You can actually lerp your speed variable that you use in Slerp.

Something like:

float startSpeed;            // This should be greater than endSpeed in order for slow down.
float endSpeed;

void SlowDown()
{
    Quaternion rotationA = Quaternion.LookRotation(randomDirection);
    float speed = Mathf.Lerp(startSpeed, endSpeed, Time.deltaTime);
    transform.rotation = Quaternion.Slerp(this.transform.rotation, rotationA, Time.deltaTime * speed);
}