Is the question that you read before you enter here, when I’m the game, and I select the player in the hierarchy, he becomes slow, and if I stop selecting him he gets the normal speed, only in the X axis, no for jump.

Here is the code, but I don’t think that the problem is there, sure is another thing.

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Movement : MonoBehaviour
{
    public Rigidbody2D rigid;
    public float speed;  // The speed of horizontal axis
    public float jumpForce; // The power of the jump
    public bool CanIJump;  // I don't use it in the code, I think that it was something that I tried in the past
    public RaycastHit2D myRay; // The Raycast on the right and under the player
    public RaycastHit2D myRay2; // The Raycast on the left and under the player
    public float distance; // The lenght of the raycast under the player
    private Vector2 rayDown; // The same that CanIJump, but I don't want to put off 
    private Movement velocidad; // The same...
    [SerializeField] LayerMask floor; // The layer that the raycasts must identify for jump
    public float TimeInTheVoid; // When the raycast are not touching the floor is, Time.DeltaTime is being added
    public float CoyoteTime; // If you press UpArrow to jump and the raycasts are not touching the floor, if TimeInTheVoid<Coyotetime, you jump

    private Vector2 MyRayPos; // It's the point where the Raycast on the right is being shot
    private Vector2 MyRayPosReference;  // It's the same as the position of the player
    private Vector2 MyRayPosMenos; // It's the vector2 which rest to the MyRayPosReference 
    private Vector2 MyRayPos2; // It's the point where the Raycast on the left if being shot
    private Vector2 MyRayPosMas; // It's the vector2 which rest to the MyRayPosReference

    public RaycastHit2D myRayUL; // It's the raycast on the left on the player
    public RaycastHit2D myRayUR; // It's the raycast on the right on the player
    public float distance2; // The lenght of the myRayUL and myRayUR
    public float jumpHelp; // The distance that is added or rested depending on, if one the raycast on the top is true and the other not

    public bool noDoubleJump; // A condition to jump, it's like a cooldown
    public float TimeToJumpAgain; // The time to noDoubleJump to be true again

    public Vector3 initialPosition;

    public AudioSource audioSourcee;
    public AudioClip Jump;
    

    // Start is called before the first frame update
    void Start()
    {
        audioSourcee = GetComponent<AudioSource>();
        transform.position = initialPosition;
        noDoubleJump = true;
        rigid = GetComponent<Rigidbody2D>();
        TimeInTheVoid = 0f;
        MyRayPosMenos = new Vector2(-0.279f, 0);
        MyRayPosMas = new Vector2(0.5f, 0);
    }

    // Update is called once per frame
    void Update()
    {

        // All this is for have the point of shot of the raycasts from the player
        MyRayPosReference = gameObject.transform.position;
        MyRayPos = MyRayPosReference-=MyRayPosMenos;
        MyRayPos2 = MyRayPosReference -= MyRayPosMas;
            
        myRay = Physics2D.Raycast(MyRayPos, Vector2.down, distance,floor);
        Debug.DrawRay(MyRayPos,Vector2.down*distance);

        myRay2 = Physics2D.Raycast(MyRayPos2, Vector2.down, distance, floor);
        Debug.DrawRay(MyRayPos2, Vector2.down * distance);

        myRayUL = Physics2D.Raycast(transform.position + new Vector3(-0.213f, 0.370f), Vector2.up, distance2, floor);
        myRayUR = Physics2D.Raycast(transform.position + new Vector3(0.279f, 0.370f), Vector2.up, distance2, floor);
        Debug.DrawRay(transform.position + new Vector3(-0.213f, 0.370f), Vector2.up * distance2);
        Debug.DrawRay(transform.position + new Vector3(0.279f, 0.370f), Vector2.up * distance2);



        if (Input.GetKey(KeyCode.RightArrow))
        {
            rigid.AddForce(Vector2.right.normalized*speed * Time.fixedDeltaTime, ForceMode2D.Force);

            if (myRay==true)
            {
                GetComponent<Animator>().SetBool("Run", true);
                GetComponent<SpriteRenderer>().flipX = false;

            }
            else
            {
                GetComponent<Animator>().SetBool("Run", false);
                GetComponent<SpriteRenderer>().flipX = false;
            }
               
           
        }

        if (Input.GetKey(KeyCode.LeftArrow))
        {
            rigid.AddForce(Vector2.left.normalized * speed * Time.fixedDeltaTime, ForceMode2D.Force);
            if (myRay == true)
            {
                GetComponent<Animator>().SetBool("Run", true);
                GetComponent<SpriteRenderer>().flipX = true;

            }
            else
            {
                GetComponent<Animator>().SetBool("Run", false);
                GetComponent<SpriteRenderer>().flipX = true;
            }
        }
        if (!Input.GetKey(KeyCode.LeftArrow) && !Input.GetKey(KeyCode.RightArrow))
        {
            GetComponent<Animator>().SetBool("Run", false);
        }

        // HERE IS THE JUMP SYSTEM 

        if (Input.GetKeyDown(KeyCode.UpArrow) && noDoubleJump==true)
        {
            noDoubleJump = false;
            Invoke("noDoubleJumpDelay", TimeToJumpAgain);

            if (myRay == true)
            {
                rigid.AddForce(Vector2.up.normalized * jumpForce * Time.fixedDeltaTime, ForceMode2D.Force);
                audioSourcee.clip = Jump;
                audioSourcee.Play();
            }
            else
            {
                if (TimeInTheVoid<CoyoteTime)
                {
                    rigid.AddForce(Vector2.up.normalized * jumpForce * Time.fixedDeltaTime, ForceMode2D.Force);
                    audioSourcee.clip = Jump;
                    audioSourcee.Play();
                }
            }
               
            
        }
        if (myRay==false && myRay2==false)
        {
            GetComponent<Animator>().SetBool("Jump", true);
            TimeInTheVoid += Time.deltaTime;
        }
        if (myRay == true || myRay2==true)
        {
            
            GetComponent<Animator>().SetBool("Jump", false);
            TimeInTheVoid = 0;
        }

        if (myRayUL==true && myRayUR==false)
        {
            transform.position += new Vector3(jumpHelp, 0, 0);
        }
        else if (myRayUR==true && myRayUL==false)
        {
            transform.position -= new Vector3(jumpHelp, 0, 0);
        }







    }
    void noDoubleJumpDelay()
    {
        noDoubleJump = true;
    }






    private void FixedUpdate()
    {
    }
}

The issue you have is that you use Time.fixedDeltaTime in Update. This should only be used in FixedUpdate.

In general you should just always use Time.deltaTime as it will automatically contain the correct values for Update and FixedUpdate accordingly.

The slow-down occurs as fixedDeltaTime does contain a fixed value so your forces are not compensated for slower framerates. When you click on the hierarchy the editor has more stuff to do so the framerate drops. → There are less frames so there is less force applied on your character.