I can't move left or right with my moving script

Hello guys ! First post here. I'm not a programer at all but for my video game art diploma i need to make some scripts in order to get a playable game. I'm working on a third person game and I had a question about this character controller script I copied from this Youtube Tutorial : Melee combat with Unity 5

My problem is, i can perfectly move on the Z axis with a decent speed, the script lauches the run animation of my character and i can move left or right by moving the camera (cinemachine brain targetting my character). But the problem appears if i want to move right or left (so when i want to use the X axis i presume) by pressing Right and Left Arrows. My character is moving, staying idle but moving but this is so slow ! I can increase the float speed in my public function and get a decent side step speed, but the Z axis is increased too and my character is too fast if i press Up or Down arrow. I would like to get the same speed for Forward/Backward AND side movements. Idid a quick video to show you the problem : Character Controller Script Issue

I think there is a solution somewhere in the script to fix that, but i'm too noob at C# to find where it is. I'll be really happy if you, skill full programers could take look at it and see where my problem is. Thank you in advance !

Here is the script (i added some lines to it ) :

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 
 public class characterController1 : MonoBehaviour {
 
     static Animator anim;
     public float speed = 10.0F;
     public Slider healthbar;
     public ParticleSystem particles;
     public float fireRate = 0.5F;
     private float nextFire = 0.0F;
     // Use this for initialization
     void Start () 
     {
         anim = GetComponent<Animator> ();
         //Cursor.lockState = CursorLockMode.Locked;
 
     }
     
     // Update is called once per frame
     void Update () 
     {
         if (healthbar.value <= 0) return;
 //
         var x = Input.GetAxis("Horizontal") * Time.deltaTime * 100.0f;
         var z = Input.GetAxis("Vertical") * Time.deltaTime * 3.0f;
         var y = Input.GetAxis("Horizontal") * Time.deltaTime * 3.0f;
 //
         transform.Rotate(0, x, 0);
         transform.Translate(0, 0, z);
 
         float translation = Input.GetAxis ("Vertical") * speed;
         float straffe = Input.GetAxis ("Horizontal") * speed;
         straffe *= Time.deltaTime;
 
         transform.Translate (straffe, 0, translation);
 
         //if (Input.GetButtonDown ("right"))
             //transform.Translate(2, 0, z);
 
         if (Input.GetButtonDown ("Fire1") && Time.time > nextFire)  
         {
             nextFire = Time.time + fireRate;
             particles.Play();
             anim.SetBool ("isAttacking", true);
         } 
         else
             anim.SetBool ("isAttacking", false);
 
         if (translation != 0) 
         {
             anim.SetBool ("isWalking", true);
             anim.SetBool ("isIdle", false);
         } 
         else 
         {
             anim.SetBool ("isWalking", false);
             anim.SetBool ("isIdle", true);
         }
 
         if (Input.GetKeyDown ("escape"))
             Cursor.lockState = CursorLockMode.None;
     }
 }