Script causes FPS Drops!

Hi, my A*pathfinding script causes hard FPS Drops. There is a huge bump in the graph in the profiler, which is because of this script:

 void PathProcess()
 {
     foreach (Enemy enemy in enemies)
     {
         if(FindPath(enemy.transform.position, player.transform.position) != null)
         {
             enemy.path = FindPath(enemy.transform.position, player.transform.position);
         }
     }
 }

 public List<Node> FindPath(Vector3 startPos, Vector3 targetPos)
 {
     grid = gameObject.GetComponent<PathfindingGrid>();
     Node startNode = grid.NodeFromWorldPoint(startPos);
     Node targetNode = grid.NodeFromWorldPoint(targetPos);
     List<Node> openNodes = new List<Node>();
     HashSet<Node> closedNodes = new HashSet<Node>();
     openNodes.Add(startNode); 
     while (openNodes.Count > 0)
     {
         
         Node currentNode = openNodes[0];
         for (int i = 1; i < openNodes.Count; i++)
         {
             if(openNodes[i].fCost < currentNode.fCost | openNodes[i].fCost == currentNode.fCost && openNodes[i].hCost < currentNode.hCost)
             {
                 currentNode = openNodes[i];
             }
         }
         openNodes.Remove(currentNode);
         closedNodes.Add(currentNode);
         if(currentNode == targetNode)
         {
             return RetracePath(startNode, targetNode);
         }
         foreach(Node neighbour in grid.GetNeighbours(currentNode))
         {
             
             if (!neighbour.walkable | closedNodes.Contains(neighbour))
             {
                 continue;
             }
             
             int newMovementCostToNeighbour = currentNode.gCost + GetDistance(currentNode, neighbour);
             if(newMovementCostToNeighbour < neighbour.gCost || !openNodes.Contains(neighbour))
             {
                 
                 neighbour.gCost = newMovementCostToNeighbour;
                 neighbour.hCost = GetDistance(neighbour, targetNode);
                 
                 neighbour.parent = currentNode;
                 if (!openNodes.Contains(neighbour))
                 {
                     openNodes.Add(neighbour);
                 }
             }
         }
     }
     return RetracePath(startNode, targetNode);



 }
 List<Node> RetracePath(Node startNode, Node endNode)
 {
     path = new List<Node>();
     Node currentNode = endNode;
     while(currentNode != startNode)
     {
         path.Add(currentNode);
         if(currentNode.parent != null)
         {

             currentNode = currentNode.parent;
         }
         else
         {
             break;
         }

     }        
     path.Reverse();
     grid.path = path;
     return path;
     
 }
 int GetDistance(Node a, Node b)
 {
     int distX = Mathf.Abs(a.gridX - b.gridX);
     int distY = Mathf.Abs(a.gridY - b.gridY);
     if (distX > distY)
     {
         return 14 * distY + 10 * (distX - distY);
     }
     else
     {
         return 14 * distX + 10 * (distY - distX);
     }
 }

I use InvokeRepeating("PathProcess", 0.5f, 0.2f) in the Awake Method. The maximum amount of enemies i am spawning is 50. It would be so nice if you could find my problem... Thank You very much!