C# path finding A+ performance problem

namespace PathLib
{
    interface IPathSquere
    {
        bool isFree();
    }
}

namespace PathLib
{
    class PathFinding
    {
        private IPathSquere[,] map;

        public Point[] findPath(Point start, Point finish)
        {
            // are the points within the map's bounds
            if (!isInMap(start) || !isInMap(finish))
                return null;

            // create priority queue
            PriorityQueue<int, LinkedList<Point>> pq = 
                                new PriorityQueue<int, LinkedList<Point>>();
            
            // setup a init list
            LinkedList<Point> lst = new LinkedList<Point>();
            lst.AddLast(start);
            pq.Enqueue(1, lst); // the priority value doesn't really matter here

            // find path, loop while 'pq' isn't empty
            while (!pq.IsEmpty)
            {
                lst = pq.Dequeue(); // get top of priority queue
                Point curPos = lst.Last.Value;

                if(curPos.Equals(finish)) // finish position?
                    return lst.ToArray<Point>(); // return the path as an array

                // add new nodes
                Point[] temp = { new Point(curPos.X-1, curPos.Y), // left
                                 new Point(curPos.X+1, curPos.Y), // right
                                 new Point(curPos.X, curPos.Y-1), // down
                                 new Point(curPos.X, curPos.Y+1) // up
                               };

                // check that they are within the map's bounds and free
                foreach (Point p in temp)
                {
                    if(isInMap(p) && // within the map's bounds
                        map[p.X,p.Y].isFree() && // free position in map
                        !lst.Contains(p)) // visited before (if so, then throw p)
                    {
                        // copy constructor
                        LinkedList<Point> tLst = new LinkedList<Point>(lst);
                        tLst.AddLast(p); // add our new point to the path

                        pq.Enqueue(tLst.Count, tLst); // add to 'pq' queue
                    }
                }
            }

            return null; // no path found ...
        }
        
        public IPathSquere[,] Map 
        {
            set { map = value; }
        }

        //help functions

        private bool isInMap(Point pos)
        {
            int rows = map.GetLength(0);
            int cols = map.GetLength(1);

            return 0 <= pos.X && pos.X < cols
                && 0 <= pos.Y && pos.Y < rows;
        }
    }
}