Rigidbody - How to make it jump and fall down fast ? Help needed.

Hello I have a question about making the character jump with Rigidbody component attached .
I tried to do it this way but the character jumps a little and falls slowly.
Does anybody knows how to fix this ? Without increasing gravity in the settings ?

public Rigidbody rb; 
public float Forward = 4000f;
 public float RL = 100f; 
bool jump= false; 
public float JUMP= 50000f; 

 void Start () { rb = GetComponent<Rigidbody>(); } 
  void FixedUpdate () 
{ 
   rb.AddForce(0, 0, Forward * Time.deltaTime);
    if (Input.GetKey(KeyCode.D))
          { rb.AddForce(RL * Time.deltaTime, 0,0,ForceMode.VelocityChange);} 
     if (Input.GetKey(KeyCode.A)) 
          { rb.AddForce(-RL * Time.deltaTime,0,0,ForceMode.VelocityChange);}
     if (Input.GetKey(KeyCode.Space) && Jump == true) 
        { rb.AddForce(0, JUMP* Time.deltaTime, 0,ForceMode.VelocityChange); 
jump = false; } 

 private void OnCollisionEnter(Collision collision) { if (collision.collider.tag == "Ground") { jump = true; } }

For me, I increase the mass before the jump (before AddForce statement), then I reduce it again in “OnCollisionEnter”.

I do not know if it is a good practice or no, but it works for me