Pathfinding while holding down the mouse button

i try to make a game where the player follows the mouse, while i hold the mouse button down so that i can move the mouse pointer and the character follows...

 using UnityEngine;
 using System.Collections;
 using Pathfinding;
 
 public class ClickToMove : MonoBehaviour {
     
     public Transform target;
     public Vector3 targetPosition;
     
 //    private Transform myTransform;
     
     private Seeker seeker;
     private CharacterController controller;
  
     //The calculated path
     public Path path;
     
     //The AI's speed per second
     private float speed = 5;
     
     //The max distance from the AI to a waypoint for it to continue to the next waypoint
     private float defaultNextWaypointDistance = 2;
  
     //The waypoint we are currently moving towards
     private int currentWaypoint = 0;
     
     
     public void Start () {
         
         //Get a reference to the Seeker component we added earlier
         seeker = GetComponent<Seeker>();
         controller = GetComponent<CharacterController>();
         
         
         }
     
     public void OnPathComplete (Path p) {
         //Debug.Log ("Yey, we got a path back. Did it have an error? "+p.error);
         if (!p.error) {
             path = p;
             //Reset the waypoint counter
             currentWaypoint = 1;
         }
     }
 
     
     public void FixedUpdate () {
         
         
         if (Input.GetMouseButton(0)) {
         
         targetPosition = target.transform.position;    
         //Start a new path to the targetPosition, return the result to the OnPathComplete function
         seeker.StartPath (transform.position,targetPosition, OnPathComplete);
         
         }
         
         if (path == null) {
             //We have no path to move after yet
             return;
         }
         
         if (currentWaypoint == path.vectorPath.Count) {
            // Debug.Log ("End Of Path Reached");
             return;
         }
         
         //Direction to the next waypoint
         Vector3 dir = (path.vectorPath[currentWaypoint]-transform.position).normalized;
         dir *= speed * Time.fixedDeltaTime;
         controller.Move (dir);
         
         transform.LookAt(new Vector3(path.vectorPath[currentWaypoint].x, transform.position.y, path.vectorPath[currentWaypoint].z));
         
         float nextWaypointDistance = defaultNextWaypointDistance;
         if(currentWaypoint == path.vectorPath.Count -1)
             nextWaypointDistance = 0f;
         
         
         //Check if we are close enough to the next waypoint
         //If we are, proceed to follow the next waypoint
         if (Vector3.Distance (transform.position,path.vectorPath[currentWaypoint]) < nextWaypointDistance) {
             currentWaypoint++;
             
             return;
         }
     }
     
 }

but the problem is that the character starts lagging while moving... i think the reason is that the currentwaypoint is set to 1 every frame...

can someone help me? :(