Why is this inconsistent?

Im trying to make a bullethell game and this is my code for the enemy shooting.

 public void Shoot(GameObject myPrefab)
 {
     GameObject bullet = Instantiate(myPrefab, transform.position, transform.rotation);
     Rigidbody2D rb = bullet.GetComponent<Rigidbody2D>();
     rb.AddForce(transform.up * 5, ForceMode2D.Impulse);
 }

}

But for some reason the bullets go a seemingly random speeds. Some will crawl across the screen where others will dash across in half a second. Help?