Issue with implementing my own slider script.

Hi, I have made my own slider script using the following code. public class DragHandler : MonoBehaviour, IBeginDragHandler, IDragHandler, IEndDragHandler {

      public static GameObject itemBeingDragged;
      Vector3 startPosition;
  
      private RectTransform rectTransform;
  
      void Awake(){
          rectTransform = GetComponent<RectTransform> ();
      }
  
      public void OnBeginDrag(PointerEventData eventData){
          itemBeingDragged = gameObject;
          startPosition = rectTransform.localPosition;
          GetComponent<CanvasGroup> ().blocksRaycasts = false;
      }
  
      public void OnDrag(PointerEventData eventData){
          float clamp = Mathf.Clamp (Input.mousePosition.y, -100f, 100f);
          rectTransform.localPosition = new Vector3 (startPosition.x, clamp, startPosition.z);
      }
  
      public void OnEndDrag(PointerEventData eventData){
          itemBeingDragged = null;
          rectTransform.localPosition = startPosition;
          GetComponent<CanvasGroup>().blocksRaycasts = true;
      }
  }

However, when I dragged the slider down using localPosition, the slider is slower than my mouse; for example, my mouse and the slider start at the same position but when I dragged it down, my mouse is below the slider. So the question is how can I fix this? Any suggestion would be appreciated. Thanks!

PS. the reason I'm not using Unity Slider UI is that I want to trigger the event OnEndDrag and when I search, it seem to have some problems about responsive or something.