Unity 2D - Jumping Isn't Smooth

Hi Everyone,

I'm new to Unity 2D, so please bear with me.

I've created a basic 2D game with UI buttons to move the player left and right as well as being able to jump. All of the buttons work but the jump isn't smooth. When holding the left of right UI button and then pressing jump the player slowly floats upwards.

The controller is below. Any help is greatly appreciated!

 private void Start()
 {
     rb2d = GetComponent<Rigidbody2D>();
     animator = GetComponent<Animator>();
     spriteRenderer = GetComponent<SpriteRenderer>();

     moveLeft = false;
     moveRight = false;
     isGrounded = false;
     
 }

 //FUNCTION - Left Button Pressed
 public void PointerDownLeft()
 {
     moveLeft = true;
     Debug.Log("Function - PointerDownLeft");
 }

 //FUNCTION - Left Button Pressed
 public void PointerUpLeft()
 {
     moveLeft = false;
     Debug.Log("Function - PointerUpLeft");
 }

 //FUNCTION - Right Button Pressed
 public void PointerDownRight()
 {
     moveRight = true;
     Debug.Log("Function - PointerDownRight");
 }

 //FUNCTION - Right Button Pressed
 public void PointerUpRight()
 {
     moveRight = false;
     Debug.Log("Function - PointerUpRight");
 }

 //FUNCTION - Jump Button Pressed
 public void PointerDownJump()
 {
     moveJump = true;
     
 }

 //FUNCTION - Jump Button Pressed
 public void PointerUpJump()
 {
     moveJump = false;

 }

 private void GroundCheck()
 {
     isGrounded = false;

     Collider2D[] colliders = Physics2D.OverlapCircleAll(groundCheckCollider.position, 0.2f, groundLayer);
     if (colliders.Length > 0)
     {
         isGrounded = true;
         Debug.Log("Player touching ground");
     }
     else
     {
         Debug.Log("Player is not touching ground");
     }
 }


 public void PlayerMovement()
 {
     if (moveLeft)
     {
         horizontalMove = -playerSpeed;
         spriteRenderer.flipX = true;
     }
     else if (moveRight)
     {
         horizontalMove = playerSpeed;
         spriteRenderer.flipX = false;
     }
     else
     {
         horizontalMove = 0;
     }

 }

 private void Update()
 {
     PlayerMovement();
 }

 private void FixedUpdate()
 {

     GroundCheck();

     rb2d.velocity = new Vector2(horizontalMove, rb2d.velocity.y);

     if (moveJump == true && isGrounded)
     {
         rb2d.velocity = Vector2.up * jumpSpeed;
     }

     if (moveLeft)
     {
         animator.Play("PlayerRun");
     }
     else if (moveRight)
     {
         animator.Play("PlayerRun");
     }
     else if (isGrounded == false)
     {
         animator.Play("PlayerJump");
     }
     else
     {
         animator.Play("PlayerIdle");
     }

 }

}