Physics2D lag on increased timescale

This scripts should be much more efficient. It controlls the entire movement and spawning of the level pieces.

 using UnityEngine;
 
 //Author: Oribow
 public class EndlessRunner : MonoBehaviour {
 
     //Controlls the speed
     public float runningSpeed = 1;
     //Controlls the direction
     public Vector3 runningDir = Vector3.left;
     //Just for testing purpose
     public GameObject prefab;
    
     //Controlls the y height of the spawned objects
     public Transform levelSpawnPos;
 
     private float outOfCameraLeft;
     private float outOfCameraRight;
     private Camera mainCamera;
     private LevelPiece firstPiece;
     private LevelPiece lastPiece;
 
     void Start()
     {
         mainCamera = Camera.main;
         outOfCameraLeft = mainCamera.ScreenToWorldPoint(new Vector3(mainCamera.rect.xMin * Screen.width,0,0)).x;
         outOfCameraRight = mainCamera.ScreenToWorldPoint(new Vector3(mainCamera.rect.xMax * Screen.width, 0,0)).x;
         SpawnTheScreenFull();
     }
 
     void Update()
     {
         if (firstPiece.PosHorizontalRight < outOfCameraLeft)
         {
             LevelPiece oldPiece = firstPiece;
             Destroy(oldPiece.piece);
             SpawnTheScreenFull();
              firstPiece = oldPiece.next;
             oldPiece.next = null;
             
         }
         UpdateLevePiecePositions();
     }
 
     public void SpawnTheScreenFull()
     {
         Vector3 position = levelSpawnPos.position;
         if (lastPiece == null)
             position.x = outOfCameraLeft;
         else
             position.x = lastPiece.PosHorizontalRight;
         
         while (position.x < outOfCameraRight)
         {
             LevelPiece currentLevelPiece = new LevelPiece(GetRandomLevelPiece());
             position.x += currentLevelPiece.HalfWidth;
             currentLevelPiece.piece = Instantiate(currentLevelPiece.piece, position, Quaternion.identity) as GameObject;
             AddNewLevelPiece(currentLevelPiece);
             position.x += currentLevelPiece.HalfWidth;
         }
     }
 
     public GameObject GetRandomLevelPiece()
     {
         //Insert your random selector here
         return prefab;
     }
 
     public void AddNewLevelPiece(LevelPiece newPiece)
     {
         if (lastPiece == null)
         {
             lastPiece = newPiece;
             firstPiece = newPiece;
         }
         else
         {
             lastPiece.next = newPiece;
             lastPiece = newPiece;
         }
     }
 
     public void UpdateLevePiecePositions()
     {
         LevelPiece currentPiece = firstPiece;
         while (currentPiece != null)
         {
             currentPiece.piece.transform.Translate(runningDir * runningSpeed * Time.deltaTime);
             currentPiece = currentPiece.next;
         }
     }
 }

LevelPiece:

 using UnityEngine;
 
 //Author: Oribow
 public class LevelPiece
 {
     public LevelPiece next;
     public GameObject piece;
     private Sprite sprite;
     public float PosHorizontalRight { get { return piece.transform.position.x + HalfWidth; } }
     public float PosHorizontalLeft { get { return piece.transform.position.x - HalfWidth; } }
 
     public LevelPiece(GameObject piece)
     {
         this.piece = piece;
         sprite = piece.GetComponent<SpriteRenderer>().sprite;
         next = null;
     }
 
     public float HalfWidth {
         get { return (piece.transform.localScale.x * sprite.bounds.size.x) / 2; }
     }
 
     public float Width
     {
         get { return piece.transform.localScale.x * sprite.bounds.size.x; }
     }
 }