How to change the speed of an action?

Question

C++ / C#

I wanted to make such a logic that with each set of points 10, 20 .... (for example) the speed of respawning of objects increases.
A simple solution is needed. I'm new to this business)
sco is the number of Spawn points
is the respawn of the enemy, it happens constantly
Second this is just the time that should shrink to cycle faster

public class spauncars : MonoBehaviour{
    public GameObject[] cars;
    public static float second = 2.8f;
     private float[] position = { -1.47f, -0.51f, 0.58f, 1.61f };
    private score SCORE;
    private int sco;
      private void Start(){
       SCORE = GameObject.Find("deletepanel").GetComponent<score>();
        sco = SCORE.count;
        StartCoroutine(spawn());
    }
    IEnumerator spawn()
    {
        if (sco % 2 == 0)
            {
                second -= 0.6f;
            }
        while (true)
        {
           {
                    Instantiate(
                    cars[Random.Range(0, cars.Length)],
                    new Vector3(position[Random.Range(0, 4)], 7f, 0),
                    Quaternion.identity
                    );
            }
              yield return new WaitForSeconds(second);
        }
    }
}