Critique my [Beginner] code. Thanks!

using UnityEngine; using System.Collections;

public class Rotating : MonoBehaviour { [SerializeField] private float speed;

private void Start()
{
    speed = 3f;
}

private void Update()
{
    Debug.Log(speed);
    SpeedUp();

    if (Input.GetMouseButton(0))
        transform.Rotate(0, 0, speed);

    else
        speed = 3f;
}

private void SpeedUp()
{
    if (speed <= 15f)
    speed = (speed + 0.02f);
    else
        speed = 15f;
}

}