Making someone happen after x amounts of seconds and only once

Hi, I am working on a game which is a remake of pong but after 20 seconds I want the ball to speed up. I have no idea how to do this BUT! This is the script for my Ball

using System; using System.IO; using UnityEngine; using UnityEngine.SceneManagement;

public class Ball : MonoBehaviour { public float speed = 12;

 void Start()
 {
             // Initial Velocity
             GetComponent<Rigidbody2D>().velocity = Vector2.right * speed;
 }
 float hitFactor(Vector2 ballPos, Vector2 racketPos,
                 float racketHeight)
 {
     return (ballPos.y - racketPos.y) / racketHeight;
 }

 void OnCollisionEnter2D(Collision2D col)
 {
     // Note: 'col' holds the collision information. If the
     // Ball collided with a racket, then:
     //   col.gameObject is the racket
     //   col.transform.position is the racket's position
     //   col.collider is the racket's collider

     // Hit the left Racket?
     if (col.gameObject.name == "RacketLeft")
     {
         // Calculate hit Factor
         float y = hitFactor(transform.position,
                             col.transform.position,
                             col.collider.bounds.size.y);

         // Calculate direction, make length=1 via .normalized
         Vector2 dir = new Vector2(1, y).normalized;

         // Set Velocity with dir * speed
         GetComponent<Rigidbody2D>().velocity = dir * speed;
     }

     // Hit the right Racket?
     if (col.gameObject.name == "RacketRight")
     {
         // Calculate hit Factor
         float y = hitFactor(transform.position,
                             col.transform.position,
                             col.collider.bounds.size.y);

         // Calculate direction, make length=1 via .normalized
         Vector2 dir = new Vector2(-1, y).normalized;

         // Set Velocity with dir * speed
         GetComponent<Rigidbody2D>().velocity = dir * speed;
     }
     if (col.gameObject.name == "WallLeft")
     {
         SceneManager.LoadScene("RightSide Scene");
     }
     if (col.gameObject.name == "WallRight")
     {
         SceneManager.LoadScene("LeftSide Scene");
     }
 }

}