Unity UI is very slow?!?

For my game I have a health bar above the enemies made with UI panels. But when I enable this, my fps goes from 1000fps to 100-200fps(see the image). What is the reason, and how can I fix this?

This is my code to create a new health bar from prefab and keep it above the enemy(note that some variables are set from other scripts):

 public RectTransform healthBar;
 public RectTransform healthBarChild;
 private int curHealth;
 private int maxHealth;
 private float maxScale;
 private Vector3 offset;
 private Transform canvas;
 
 void Start()
 {
     canvas = Global.canvas;
     healthBar = Instantiate(healthBarPrefab).GetComponent<RectTransform>();
     healthBarChild = healthBar.GetChild(0).GetComponent<RectTransform>();
     maxScale = healthBarChild.sizeDelta.x;
     healthBar.SetParent(canvas);
 }
 
 void Update()
 {
     healthBar.position = Camera.main.WorldToScreenPoint(transform.position+ offset);
 }
 
 void onDamage(int damage) // is called when enemy gets damage
 {
     curHealth -= damage;
     Vector2 curSize = healthBarChild.sizeDelta;
     curSize.x = curHealth / (float)maxHealth * maxScale;
     healthBarChild.sizeDelta = curSize;
     if (curHealth <= 0)
     {
         die();//I didn't include this(and some other functions) as it is not necessary
     }
 }